I seem to be having a hard time finding a coat that fits my shoulders. I measured yesterday and I’m 14-15" shoulder to shoulder. I tried on an 8 slim GP and it fits pretty well, shoulders might have been a tad wide and it was a tad long but otherwise it was pretty good. Only reason I didn’t buy it was because it was a little out of my budget. I’m watching a 0S RJ Classic (Essential Collection) on ebay and I think I want to buy it, but the lady selling it said that the shoulder to shoulder length was 16". I also read somewhere on this forum that GP tends to have smaller shoulders than RJ? What do you guys think? I’m small, not quite 5’ 1" and about 100 pounds. Whatever jacket I get pretty much has to be short length. I’m just worried the shoulders won’t fit and I don’t have time to try on an RJ coat before this listing ends, and it’s a good price so I want to get it! But will the shoulders be wider than the 8 slim GP that I tried?
I can’t answer your question about the shoulders, but I recommend that you try on some children’s coats. They tend to be less expensive.
My 21-year-old daughter is about your size (actually, she is shorter), and she wears an RJ children’s 18 short. You might be able to get away with a 14 or 16–and you’ll save money. Is there a tack shop near you wear you can try on coats?
GP is generally actually wider than RJ.
I did try on a bunch of coats, children’s coats don’t have the right cut for me 
The waist is always too big. A little too boxy. I ended up ordering the 0S off ebay. Hopefully the shoulders fit! The GP coat I was looking at was only $75 (consignment) but I’m a broke college student so I wanted to find something a little cheaper. $75 is almost a whole paycheck for me
If this helps at all- I wear a 6-8 in RJ classics and wear a 12 in Grand Prix. Grand Prix don’t look as good on me, they’re a bit boxier cut and the RJ’s look more tapered through the waist line. Both are lovely brands, I hope yours works out for you! If it doesn’t you can always resell 
The grand prix coats are much nicer than the RJ essential line. However, saying that, the RJ essential line is quite nice for the price, especially if you only plan on the occasional horse show.
But my advice, if you don’t think you will change shape much - invest in a good show coat. The grand prix’s and marigolds (and the pricier RJ lines) are generally very much worth the investment and you can often find them used in your size. If it means skipping a show, or a class at a show, in the long run it will be worth it because you wont need to replace your jacket in a years time.
FWIW, I have a marigold jacket going on 15 years (and several other marigold jackets at least 6-7 years old). It looks brand new, and I have done nothing but neglect it. The Grand Prix shadbelly is at min 10 years old and is also holding up very well.
With the newer soft shell and stretch jackets on the market, the older classic but well made jackets should come available in your size and price range on ebay or your local consignment store.
I have found with any item of clothing in life it is incredibly hard to find something that fits perfectly right off the rack. Buy a coat that fits the biggest or widest part of your body (sounds like shoulders in your case…and mine) and have the rest taken in by a good tailor. Even an item of clothing that is not very expensive can look amazing if it fits just right. As far as brands… I don’t think you can go wrong with a good quality fabric regardless of the brand. I have Grand Prix and Tailored Sportsman hunt coats and an RJ shad. All have been tailored. All look nice IMO.
I wear a 0S in RJ classics and a 10reg in GP. The 0S is still a little wider then my 10R GP.
I do agree that the fit through the shoulders is much more important than a bit of looseness through the waist, which you can always have taken in.
OP, try children’s sizes. I’m built very similarly to you and I wear children’s coats. GP are the only ones that work for me; RJs are too boxy for my frame, but a GP child’s 14 works quite well for me. I have to get the waist taken in a little, but not too much. Plus, the added bonus is that children’s coats are cheaper!
