smartpak has the best prices (smartpak.com) for a lot of attire.
schooling: you can really wear anything, breeches, shirt, belt, tall boot socks, boots/half chaps or tall boots. my everday riding outfit usually is: tailored sportsman trophy hunter breeches, some type of sun shirt (shirt w vents) (eis brand or tailored sportsman usually), a random belt (tailored sportsman, tory, c4 is rlly affordable), some random tall socks, my favs are noble outfitter peddies, & then any boots.
hunter show: show shirt (essex, tredstep, kingsland, etc.), tan breeches (& tall boots) or tan jodhpurs (& paddock boots w garters), a black belt (w black boots) or brown belt (w brown boots)
jumper show: any nice shirt really, again i wear sun shirts for this sometimes, tan breeches (read above), boots (read above). jumpers is much more relaxed w the attire.
and ofc a helmet. if its a rated show it has to be an approved helmet (look it up)
Dover is a good place. Try second hand too. :)
HUNTERS: white pad with saddle, boots not polos and braids (for the horse)
JUMPERS: More relaxed, saddle pad (and half pad if needed), saddle, boots or polos.
Wear any shirt. For pants wear breeches. For shoes wear paddock boots and half chaps. I usually get my clothes from my barn because they sell tack there. Also try eBay because stuff is really cheap there. Stores like dover are a bit more Pricky but smartpak is in between.