You have a few options here. You can buy ammo online. Or you can get into reloading. Both cost more up front (assuming you buy in bulk online), but ultimately will give you more ammo at lower cost.
I’ve heard a Viper-45 and was very impressed. Good choice.
Now, the other option you have with a MAC10/45 is to convert it to 9mm. This is actually not all that difficult, and you can swap back to .45 whenever you want.
You need a 9mm upper, bolt, and a magwell adapter. You will likely need a different mag catch too. Here is a set from Cobray.com that has most everything you need.

I did a similar setup, but pieced mine together instead of ordering a “kit” like this one. Same end result, though. You want to be aware that in the 9mm, there is a double-feed bolt and a single-feed bolt. Either will work, but you have to match the type of magazine you want to use to the bolt.
The double-feed bolt uses the original Walther-style (expensive) 9mm mags. The single-feed was designed for the zytle M11/9 mags. You can also use the metal mags from Recon Ord with this type of bolt.
Caliber issues aside, I’ll agree that in stock form, the MAC10 is not the easiest gun to shoot. As noted, a silencer or just a plain barrel extension makes a big difference. The added weight up front helps. Having somewhere to actually hold with your off-hand makes a big difference.
Oh yeah - with a silencer, you need some kind of can cover so you can hold it without burning your hand. The Viper has one built in already. On mine, I would use a lace-up leather and nomex cover, but which has been out of production for a while. You can find new covers at http://www.subguns.org/products/covers/. I haven’t tried these, but they look like they should work well.
Once you get a silencer / barrel extension, look at the stock. The wire stock tends to slide up off the shoulder when shooting. I looked around for several months trying to find an original wood stock for my MAC10. I finally gave up and built my own, based on pictures from an old RPB catalog. I like the way it turned out, and it made a world of difference in how the gun handled.


