goldfish are very messy fish, so please be sure that you are doing daily 50% water changes (as necessary based on your water test results).
As already mentioned, the best source of bacteria is substrate or filter media from an established tank.
The second best source is a reliable bacterial starter product, I would personally put my money on Tetra Safe Start Plus or Seachem Stability. The dosing directions are different on both products since the product is different. If I understand correctly, TSSP is a live bacterial product, which is why it works slightly faster but it is less shelf stable and prone to losing potency if exposed to temperatures outside room temperature. Seachem stability contains bacterial spores, which means that it takes a bit longer to work (the bacteria have to "wake up" and get out of spore form), but it has better shelf life. Other products that contain bacteria are API quick start as mentioned, and aquaeon Pure bacterial starter. Your fish store should carry some type of bacterial starter, and if not, I would order seachem stability online (just because I think it ships the best out of all of these products, though I believe
@mrsjoannh13 ordered her TSSP online and it arrived just fine). Petsmart sells both stability and TSSP, as well as the other products listed if they happen to be out. When choosing a product, look for something that is described as "quick-starting the cycle" or "establishing the nitrogen cycle" instead of "contains beneficial bacteria that supports the cycle" or "contains bacteria that break down fish waste" there are products with similar descriptions (such as API stress zyme) that contain different bacteria that do not help the nitrogen cycle directly.
The third best source of bacteria is live plants. Live plants also have the added benefit of taking up ammonia (and to a lesser extent) nitrite and nitrate directly out of the water column, reducing the stress on your fish. Because you have orandas, I'm not sure how many plants woulda actually survive if they are as herbivorous as I think they are, but hornwort is generally a plant that most fish find unpalatable. It is usually easy to find in pet stores, except in areas where it is illegal due to it being a potentially invasive species. I got my hornwort from petsmart. Just wash it well, since it often comes with snails.
Finally, since you are doing a fish-in cycle, I would recommend investing in a product like Seachem Prime or equivalent. Prime I would describe as the industry leading water conditioner, since it not only dechlorinates, it also detoxifies ammonia and nitrite (without removing them) so it makes the water safer for your fish without altering your cycle.