DrDan,
This is my opinion derived of experience but nothing at all like research. The problem you are having is common. It is my opinion that it is caused first by grips that are too small in diameter and second by grips that are made of rock hard material. Both are the case with stock Sportster grips - they are small in diameter and very hard material. You will do yourself a great favor by purchasing a set of grips that are just a little bit bigger around and with soft inserts.
Here are the ones I put on my bike. I did it for the very reason you mention, the stock grips really hurt my hands after just an hour or two - and by the way, its much worse when you wear gloves:
Notice that the rubber pads are raised which keeps your hands off of the metal part. That is important because direct hand contact with chrome is even more uncomfortable on hot humid days than the cramps you get in your hand from hard rubber grips.
Anyway there are a zillion different grips available and as long as they are 1" in diameter (ID) they will all work. For the most part they are interchangeable although on the Clutch side Harley uses a slightly more complicated locking system than other bikes. With the Harley system the clutch stand and switches lock over top of the end of the grip whereas generic grips simply push up against the controls when mounted. Its not a problem.