I'm doing Equetech's Canter for a cure with a group of mates and the theme this year is PATRIOTIC!! Ideas and shops of what I can buy for me and my horse!!!
Decathlon or Donnay sport ; just buy cheap oversized t shirts in red /white /blue and add accessories -legwarmers ribbons etc - in the other colours rather than spending on specifics.
Make a budget, stick to it, and anything left over put in the charity fund.
(I saw someone spend over £100 on Joules pink stuff for a charity ride saying she'd never wear it again - missing the point or what?)