.22 rimfire is in short supply, but most other calibers, can be found without any trouble.
My favorite online source is SGAmmo.
Need some 5.56/.223? They have plenty at reasonable prices. They have a decent, if not large selection of 9mm ammo (although they are currently out of 9mm ball).
Much of their pistol ammo is self defense types, such JHP. I bought 200 rounds of Starfire 125 grain JHP +P .38 special and 200 rounds of Hornady Critical Duty 135 grain 9mm. I now have enough ammo for the home, and enough to hit the range a few times as well. I load the .38 special in my Rossi .357, and the 9mm in my Marlin Camp 9.
They also have a huge selection of 12 gauge ammo, and I highly recommend Rio as a best buy in first rate shotgun ammo. I bought 100 rounds of 2 3/4" Rio Star (1410 FPS) 1.125 oz rifled slugs at $3.50 per box of 5. Excellent stuff, shoots as accurate as the big name US made ammo.
Here's the link....
http://www.sgammo.com/