It really depends on the show.
Some small shows are nationally sanctioned in which case you are subject to their attire rules which usually require a jacket.
The ones which are unsanctioned are generally far less picky and as long as you are "properly attired" and not in shorts they will be fine.
As far as for the smaller shows, even in the nationally sanctioned ones, I have no issues with someone simply wearing a black or navy sweater without a hood, it doesn't even really need to be a blazer, but check with your show organizer anyways.
They say money doesn't buy happiness -- well happiness doesn't buy horses!