Criss you sound just like me. As of a month ago, I had no tall boots, no breeches, no dressage coat, and no show shirt, and was think that nothing would fit me and my large hips and calves, but small waist and chest. I ended up buying the largest size of Hispars, and now I am having them altered to fit by having leather added and removed as necessary. I found that the on course side zips I bought fit great, so I am thrilled about that. As for the dressage coat, I just went into my local tack shop that has their own brand of coats, the Hawthorn line and Libertyville Saddle Shop and found one that fit in less than 5 minutes, and the best part, it was only $135! I am going to pick up a Kerrits ventilator shirt and a stock tie this weekend at another tack shop in my area to complete my outfit. So here is my advice to you:
Buy what fits the largest part of you, and tailor the rest. If a coat fits your shoulders, you can have the rest taken in to make a custom coat, for a fraction of the cost.
If a pair of boots comes close to your size, buy them and try them. If you can almost get them on, just have a zipper put in, or have them modified by a leather repair place. It may seem like it costs a lot, but it will cost a lot less than custom boots, but will fit like them, and in a lot less time.
Good luck with your search. I am sure you can find what you need for the show season if you look hard enough!