Betta Buying Advice For A Newbie

Hi TFF users, I'm new to the world of fish and looking for some advice. I'm planning on buying a Betta for my son's birthday and I need some advice... is it OK to buy online? Can anyone recommend someone to buy from? I was looking at getting an orange veiltail, are they relatively common? My son is very keen on the colour, but more flexible on the type! 
Once the fish arrives, what should I look out for? The tank is already set up so I was going to wait a week before ordering.
 
So many questions! Thanks everyone!

Personally I like to see and observe a fish before I buy it so while a fish (especially a betta) will ship you get what they send you. If that's okay with you then online will work. Since I'm in the US I won't be able to give you a UK online resource. 
 
Betta are very popular and a great fish to start with but there are some misconceptions about them. This thread http/www.fishforums.net/index.php?/topic/423364-betta-splendens-caresheetshould do well to get you started and if you're lucky Wildbetta will pop in and share some wisdom with you. 

Orange veiltails are not too common to come by but I have seen them a few times in my local pah and purchased one so I would def keep looking or ask if they can order you one in? buying online comes with it's risks of not getting the fish you want, happened to me a few times, to the high costs of shipping and some fish do not travel well, plus it is getting colder in the UK now even with a heat pad the temp still drops quite a bit if the box is too well sealed as they need oxygen to work.
Here is my boy sadly no longer with us 

 

My orange half moon came from Maidenhead Aquatics. I have two reasonably close branches and one not so close, and the nearest gets a lot of different coloured bettas in. It is worth checking all the branches in your area.
 
Just to check, you say the tank is set up and you want to wait a week before getting a betta. Have you cycled the tank? If you don't know what I mean read this. If you get a betta without cycling the tank first, you'll need to read this and this

Thanks everyone... and particularly re: cycling... I had no idea. The shop I bought the tank sold me Fluvial cycle Biological Enhancer, but it sounds like I need to set it up further in advance than planned. So glad you brought it up @essjay.
 
Thanks for the advice everyone!

Not many shops mention cycling when you buy a tank. That's where forums are so useful
 
 
I forgot to add a photo of my orange betta to show you what you can find if you look hard enough.
 

Beautiful @essjay!
I just bought a betta log made by Zoomed on Amazon, they had great reviews, but I'm concerned it's a bit rough around the edges so I think I'll return. Sad, as it sounded like they really enjoy them, not sure if I just got an unlucky purchase!

Ah don't send it back!
Just use some sand paper and smoothen it down. I usually do this with ornaments and things. No need to send them back

Chat to the folks in your local fish shop.  They may be accommodating and order you a fish you specifically want.
 
Alternatively if you do wish to buy online there are several resources in the UK (I've not used any of them BTW so cannot vouch for quality or service).
 
http/www.aquaticstoyourdoor.co.uk/Photographed-Fighting-Fish/
 
http/www.discountfish.co.uk/TROPICAL-FISH-ONLINE.html#Live-Tropical-Freshwater-Fighting-Fish
 
http/www.tropicalfishbypost.co.uk/siamese-fighter.html
 
The top one offers photographed fish.  The others would be pot luck.  If I were you I'd get on good terms with the LFS and see if they can help you out.