You make your own! Well,kinda....sure you can buy gas engines from a store,but its WAY cheaper to buy used weed wackers,leaf blowers,chainsaws etc and rip out the engine and convert it over to r/c use. These engines are almost the same as your store bought gas engines,but require some stuff(which is the hardest part) such as prop hubs,sometimes mufflers,and sometimes stand off engine mounts. But some of us,like me,make our own. As flaminheli said though,most guys dont put a "coverted" engine in a plane less then about 70",but it can be done for some planes. Its great though as it adds a whole new hobby to r/c flying. And compared to store boughts engines and the glow fuel...its dirt cheap,just requires some added skills. "Gas" engines seemed intimidating to me when I first got into this hobby,but their not too bad. And their MUCH more realiable then most glow engines.

rcu gas conversion engine forum.... http://www.rcuniverse.com/forum/forumid_92/tt.htm

regular rcu gas engine forum.... http://www.rcuniverse.com/forum/forumid_142/tt.htm