The two that come to the top for .25/.26 engines for Scale Combat are the OS.25FX and the Irvine .25. The FX can be had from Tower Hobbies for $84.95 and the Irvine can be had from Just Engines in England (www.justengines.unseen.org) for about $75 or less.

A note on Just Engines:
They are super people to deal with. They often have the Irvine engines on sale for reduced prices and the shipping charge is minimal. I bought two Irvine .25s from them, the engines arrived within a week and the cost was $143.00 total. That breaks down to $71.50 each including shipping.

HTH,
D