Cashel makes one- the rise is huge but since you need 5 inches, probably not too huge for you. I forget where I got mine but it was on one of the mainstream tack store websites.
Or you can get a skito pad and create a rise in the back with the inserts (whihc is what I did since the cashel was too much rise)
There is something about the outside of a horse that is good for the inside of a man.(Churchill)
I have several. If you are on a tight budget I like the Roma pad that Highflier linked to the best, because it stays in place and is not as thick in the front portion as the Cashel pad. It's important to take this into account because it will affect your saddle fit. If you do not have the room to add a thick pad under the saddle it will end up tipping even more and of course it will be too tight in the wither.
You do have to be very careful with bridging. If your saddle's tree does not have enough curve in it to follow the shape of your horse's back you are going to fun into this problem. I have a swaybacked TB and went through this whole process with him. The Roma pad was probably the best one I used because it is also shaped to handle a very high wither, whereas the Cashel pad is flat.