I need a scope for my airsoft sniper...?

I need a scope for my airsoft sniper gun. The gun i have is a MP001 Sharpshooter. It isn't a very popular brand of a gun, and i cant find a scope that would fit it. I bought one scope and it was too small to fit the gun. I am wondering how i can find a scope to fit my gun. The scope mount is about 5 1/8 Inches long and 3/4 of an inch wide. I was wondering if anyone could help me find a scope that will fit my gun. Also i don't want it to be much more then $20 dollars, because it is just my airsoft gun. Thank you for your help!