Answer to "Can I use a 9mm 115 grain"Again, the easy answer is NO. Your "9mm" (also known as 9mm luger or 9mm parabellum) is 19mm long and will not chamber. Additionally, even tho they are both "9mm", 9x18, also called 9mm Mak, is a larger diameter. 9x17, also known as 9mm kurz, 9mm browning, and .380acp will work in a 9x19 pistol just like .38 special in a .357 or .22lr in a .22 mag, but the smaller diameter will make it highly inaccurate in a 9mm Mak. The correct caliber should be stamped on the slide and "alternatives" are generally a bad idea.
Answer to "What kinds of ammunition can I use in this gun?"The correct caliber should be stamped on the side. While you might be able to use .380 in a 9x18, it will be inaccurate, and you would certainly NOT want to try 9x18 in a .380. Update (12/10/2016): If it is 9mm Mak, you can usually find it at gun shows or online at gunbroker.com