Avoid P@H. Or at least don't listen to anything they say as the shop assistants are just that, shop assistants not fish experts. They are trained on how to sell and nothing else. This shop will also question you about your tank - don't mention fishless cycling as they are told not to believe in it - and will only sell you a set number of fish per customer. This is company policy, even if the customer has a newly cycled 500 litre tank they'll still only be allowed to buy half a dozen fish.
MA can be variable as they are run on a franchise basis, each with a different manager who has control over the store. Some are better than others. If Littlefish vouches for the one she uses, take her word for it.
Different branches of MA stock different fish. It is up to the manager what they stock. They'll all have the same bread and butter fish like neon tetras and harlequins, but the less common fish vary from store to store. The chain has a list of fish they can sell and it is up to the manager which fish off the list his/her store has. The branches in any area will have fish from the same wholesaler, so any infections/weak fish at the wholesaler will end up in all the branches.
Independents can be hit and miss. Some look good but are not; others look a bit rough but have good stock. Some specialise in, say, Rift Lake cichlids, others have a limited selection of fish in general.
And bear in mind two of my local-ish shops. One has pristine tanks with nice looking fish which don't live long. The other has tanks full of decor and plants and with gravel on the bottom, all covered in algae. They don't look very attractive. But this shop quarantines all new fish for a week, rather than overnight, and the fish I've had from there have been healthy.
My best advice is to visit all of them, but leave your wallet behind. Go window shopping, see what you think of the shops, and see what they all stock. Make it a day out
And take a notebook, whether paper or electronic or you'll forget what you've seen where.
And once you find a shop you like, stick with them. If they don't have what you want, ask if they can get them for you.
When adding the first fish it doesn't really matter where you get them from, though personally I like to get my fish from the same shop. It's when adding new fish to an established tank that the danger of disease can arise. In this case, a quarantine tank is a good idea.