There are two standard sizes for snaps that I know of: Snap 24s and Snap 20s. Both are available at Tandy Leather
. If you're near a store you can take your bags in and try them on their snaps.
s are larger, and probably what you need. The inside "bump" part, or male, is 0.377" OD. The female receiver is the same.
Snap 20s are smaller and lighter, with the male part at 0.260", the female part being the same.
If the snaps are pulling through the leather you should reinforce the leather before replacing the snap. Remove the snap part on the leather that's pulling out. Get a scrap piece of thick-enough leather, put a hole in the middle, and use contact cement on the inside of the saddlebag and the non-shiny side of the leather patch, and glue it in place. Then replace the snap part.
Even if you can't find an exact replacement, it'll be easier to remove both the male and female parts and replace both. Drill them out and carefully use needle nose pliers.
Tandy Leather has a kit that contains snaps (10 brass and 10 chrome), a setter and anvil, and instructions, for about ~$10. It's better to go with something standard because if you need to do the repair again, or to repair any other snaps, you'll be better prepared.