Most likely there'll be a festive in that city/town where boutique is, where we can play games or shopping by using Festive Coins. I think the basic assumption is using Poke Money to buy Festival Coins (hence the Game Corner-like feature). However the clothes sold may be very expensive, which is why I recall some quests give you a lot of money (Pyukumuku quest gives you $20 000 for completing it), or maybe other quests that gives you Festive Coins.
That aside, do you guys think that the boutique shop will be only at 1 area, which is that specific city/town? Since the boutique uses Festive Coins I can't see any other city/area having a boutique and not using Festive Coins, but using normal Poke Money instead. That would be odd, cause that will make Festive Coins a bit redundant.