Depends what sort of fish you're after. Wildwoods for example is a very good retail store (perhaps the best in England) for rare, wild-caught and oddball fish. I get a lot of stuff from them, and Keith Lambert, who runs the fish section of the business, is much respected in the trade. Other stores have different strengths.
In any event, you ideally want a store with a good reputation (ask around) and a guarantee of some sort on the livestock. Be prepared to pay a bit more for good quality livestock sent online, since shipping fish quickly and carefully is going to cost a bit of money. Talk with the retailer about delivery: you want to be home when the fish are shipped, so if you can fix a date and time, that's enormously helpful. Think carefully about whether it makes sense to mail order certain species. Some fish ship well, but some don't, and for those you really are much better off sourcing them locally if you can, or else travelling out to the store and carrying them home yourself.