@Sotbas
You do not give your location beyond USA. This makes it hard to suggest anyplace which mighy be local to you. The rare the fish one wants to buy, the more likely it can be to locate a seller. Most of the fish what I work with are not sold in retail stores. One needs to find more specialized sellers to find such fish.
Over the years I managed to find the sellers I needed to discover. But I am now at the end of my serious hobby years. The last big fish purchase I made was in early 2020 and it was an import deal, so most of the fish were got to be resold. However, I got 25 fish for myself at a price which is really great. All we had to do was to buy 350 fish. But there are still good places to shop. They are just not as easy to find. You have to work at it.
One good place to start is a local fish club. Some members breed fish, others have been doing fish for so long they know the best places locally, if they exist. As a general rule, fish stores will be a better option than pet stores where fish are just a part of what they do. I have two stores within about a 30 minute drive which are superb. But the prices they change are pretty high. I have shopped at one but not the other. Most folks who have been in the hobby for a long time tend to be in a fish clud, Such folks know the best places to shop based on years of experience. And they will know all the stores within a reasonable drive where one should shop.
I have not bought there in ages but AquaBid used to have some good sellers. And there there is feedback. Usually the higer rated sellers are a decent risk. A lot of them breed what they sell.
One good supplier out there is Wet Spot in Portland Oregon. Most folks do not realize that this place is the retail arm of a big import wholesale operation. They have a great selection and they know how to ship.
Finally, there are a number of weekend fish events. Some of the best contacts I have made have come from attending such events. Usually the vendor room will have a number of good sellers. Events like that are where I have been able to meet and talk to some of the most well know experts in the world of fish. They can be scientist, collectors, breeders and hobby sellers as well as some of the more commercial operations.
Basically, it takes some effort to discover the best sellers for the fish in which one is interested. Here is one important factor to consider when trying to determine where to buy. I breed rare expensive plecos and I sell them to pay for my hobby costs. I take great pride in the quality of what I raise and I sell at fair prices. I stand behind what I sell. There are plenty of hobbyists out there who have a similar attitude.
But a retail operation is a business. When this involves live animals there are more risks to both sellers and buyers. I will spend a bunch of money to buy the fish which I will try to spawn. Often they start small and I have to grow them. But once I have them spawning and can sell enough offspring to recover my costs, the fish born in my tanks are pretty low cost to me. A store or retail online operation mostly buys the fish that they sell. Given these facts if I lose fish in transit I mostly lose the income from the sale. But when a retail operation buys stock they put out cash. If they lose fish they lose money. So they are often motivated to move stock as quickly as possible. The longer a store has to hold fish before they sell, the greater their cost for the fish becomes due to rent, electricity, care, feeding etc..
Most sellers want to be honest and not to have unhealthy fish that die before they sell them or soon after. Unfortunately, the economics involved may influence how sellers deal with things. Reputation matters. So, one good way to find the good sources for fish is from other hobbyists who have been around for a while. Such people usually have multiple tanks ans an assortment of species. And that brings us back to fish clubs where we all meet each other on a regular basis.
Sites like this one can also provide decent sources, The problem is there is no filter here which tells users whom they can ask for advice. The fish specis bought by folks just entering the hobby tend to be the easier less expensive ones. It is one thing to buy a fish for $5 and have it die than if you are spending $50 or more per fish. The more expespensive a fish is, the more important it is for it to be healthy. For sellers, replacing a few inexpensive fish is one thing, it is another if a $150 fish dies in transit or soon after one gets it in their tank.
It took me years to get some of the fish I wanted which are very pricey. The delay was caused by the amount of time it took me to locate sellers from whom I could expect to get healthy fish and correctly identified as well.
Finally, while it is not a guarantee, but the number of tanks a fish keeper has as well as the number of years they have been keeping fish are good indicators of their level of knowledge about the fish but also where to acquire them.
As far as what we put into our tanks besides the fish and the water, ir is important to know we are not anything that might be harmful. If we are not 110% sure a piece of wood, a rock, substrate etc. is safe, then do not use it. The problems are usually something we cannot see. That pretty rock on the side of the road may have weed killer or exhaust fumes on it. Thay lovely piece of wood may not be so great internally. A lot of this is just common sense.
