Gerbils are perfectly fine for ball pythons, being one of the kinds of rodents balls eat in the wild. However, pre kill them before feeding them to your ball python, this will ensure no change of injury to your snake. One thing to keep in mind is ball pythons may decide they like gerbils more than anything else and refuse anything else. Balls can be very picky eaters, though some will take whatever you offer no matter what it is.
Something you can look into is buying a couple rats (one male and 1-2 females) while young and grow them up and breed them, this way you will have all the rats you need of the size you need. Just freeze the appropriate size you need and thaw one when ready to feed your snake. If you get large litters and have lots of extra, you can either get another male rat (or just grow up one of the males from a litter) and you can keep the two males together and not breed the females for a few months, till you have used up your frozen supply of food. Or you can sell excess to other snake owners in your area or even back to the petstore if they will take them.
If keeping rats is not to your liking or you just are unable to do it (alergies, space etc), ordering 20-25 frozen rats of the appropriate size can make things easier as well, either through the petstore itself, or an online supplier (canadian mouseman (I think that is what they are called) exotic pro, Port Credit Pet Center, etc)
Personally I breed my own rats, and with 6 snakes to feed, I don't have alot of extras to sell to others (though right now, two of my snakes are too small to eat rat pinkies, but I kept mice long enough to stock up on mice pinks and fuzzies as well as some small hoppers. By the time I use all this, the cornsnakes will be old and big enough to take rat pinks). Going through 4 weaned rats a week goes through a litter pretty quick and since it does take about 4-5 weeks to get the rat babies to the size I need, I don't have a huge stockpile y et, even with three females, but have enough to keep everybody fed on feeding day.
If you just have the one snake, one male and one female should be enough but if you find you are running out before the next litter is born or large enough, you can always let one of current litter females to grow up and start breeding her. Though she will be bred by her father, unless the two rats are already heavily inbred in their past genetics, there really shouldn't be any problems. If you can find a male from a different city or petstore that doesn't get their stock from the same source as all other petstores in your area, this helps provide a 'clean' genepool to start. I have bred first, second and third generation 'daughters' to their dad (or grandfather, depending on exactly which generation the female came from) without any noticeable genetic defects in the offspring or lowering of birth rate. However, it's always good to enter in 'fresh blood' from time to time, either by replacing the male for a completely unrelated one every year and half to two years or starting with a fresh completely unrelated group of females or both.