There is a RCU forum devoted to glow [aka nitro ] engines Glow Engines - RCU Forums

A good general purpose fuel for the average RC glow engine will have around 16 % synthetic oil 2 castor oil 10 % nitromethane and the balance methanol.

But there are engines which need more nitromethane eg the small Cox engines and others which need less oil eg the big Super Tigres.

As for buying an engine well there are some specialist suppliers still around eg Just Engines Online Ltd

Buying a used engine is gamble buying one off Ebay is a big gamble. Lots of over priced iunk on EBAY. I just had a quick look and out of the first 100 there was nothing worth buying unless you were a collector

It would help if we knew what sort of engine size you were looking for and what sort of plane