Jean Perry has put together several books for the horse owning do-it-yourself crowd. Make Your Own Riding Clothes has clear explanations and diagrams describing the process of altering patterns to make jackets, jodhpurs and other apparel. You'll need to have some idea how jackets and pants go together before attempting to alter the patterns. A bit of sewing experience would be helpful before setting out.
Make Your Own Riding Clothes - Buy It Now
If you just want to follow a pattern without going through the steps of altering the best source for sewing patterns is SuitAbility. I've used several of these patterns with satisfactory results. I did find they fit a bit large, but that was solved by nipping in the seam allowances. They also have patterns for horse blankets, hoods and just about anything else you can sew on a sewing machine.
A young friend of mine gave me this tip for turning a men's dress shirt into an English riding shirt. You'll notice that the collar on a men's shirt has two parts. Carefully cut off the top portion of the collar. You'll now have the winged collar portion and a small band still attached to the shirt. Sew a tiny hem along the cut edge of the winged portion. Sew a button on one end and a button hole on the other. (You could use hook and loop fastener too.) Now turn the collar upside down and button it behind your neck so the 'wings' of the collar are pointing downwards. Just add a pin to the front and you have a traditional English style shirt.
Comb second hand stores for shirts, pants, jackets and vests that can be turned into riding clothes. You many even find 'real' riding clothes. I once found a beautiful pair of leather riding boots for $8 and a friend of mine unearthed a nice pair of schooling tights.
Stretch tights can easily turn into riding pants, as long as they are the right color. The seams of stretch pants like yoga pants can be taken in and turned into riding pants. If you want to wear white jodhpurs though, make sure white tights are not see-through. You want to catch the judges eye in the show ring, but not because the color of your underwear is showing through.
