How hard is it to get a hunt coat altered?

Any competent tailor should be able to do just what you're asking, especially if the necessary alterations aren't too large. VERY easy to have the waist nipped in, and taking in the shoulders isn't really all that involved, either.

It should be fairly easy for a qualified seamstress or tailor. Basically, you go to a fabric alteration shop with you and your jacket, they will measure the jacket on you, and bring everything in to the correct size for you specifically. They should even be able to make the shoulders a little smaller and some areas a wee bit bigger if necessary (though by wee bit, I mean about 1/8" bigger which really isn't much). It also should be fairly cost effective as well, normally around $25.

since it is big and you want it to be taken in, it should be no problem at all. Find a local dry cleaners (that would be cheaper than a tailor) and ask them if they can altor it. Make sure whoever does it knows that it needs to have give so that you can move around and ride comfortably in it. It should not be expensive. I would say that it couldn't be that expensive. and shoulder pads should be fine as long and they are suck in there well (so they don't fall out while you are riding).

Bring it to a tailor and see if they can fix it without destroying it. If you find a really good tailor they can fix just about everything to make it fit. Ive had to bring breeches in before cause I lost weight and they where able to fix them pretty well. You couldnt even tell they where altered until you look in the inside.