Asda for a work shirt, smart and cheap. Or a school shirt for the kids. If it is warm weather you can get away without a jacket or waistcoat. But you should wear a tie if you are going without, I think it looks smarter. A waistcoat from Sherwood that is thinner/spring weight in a neutral shade (forest green/brown/beige) to tie in your trousers (depending on colour you go for) Dark or light, you can get away with both. They say dark for pale legs and light for dark so the judge doesn't get the legs mixed up! but they dont really seem to mind.
Hair, workmanlike. Hat for kids (helmet/velvet) you can wear a stock hat or a cap for you if M&M (some shows say Adults must wear a hat, others don't specify) but children always will need to. Smart shoes, suitable for running and you can get away with dark trousers (again look in Tesco/Asda) or beige Jeans (Matalan) Robinsons do some own brand cheaper Riding/boot cut trousers in beige.
To be honest I have found that as long as you look smart and the pony moves well then they look at the horse not the handler. If you teach the ponies to walk out well, stand square and trot on command, nothing looks worse than a horse being dragged and it is surprising how difficult it is to do in-hand well. I thought it would be easy but after my first show season of "comedy legs" I did a bit of "Ginger biscuit" training and it was 100% worth it, lots more rosettes! Hope this helps a bit.
Andrea