I'll take a look around, but depending on where you are, the shipping can cost more than the engine!
If your willing to spend the $ you can still get brand new ones. smallenginewarehouse.com just had some 5hp I/C engines. The I/C has the correct carb/tank type that you're looking for. Unfortunatly they sold out about two weeks ago. Don't confuse this engine with the Briggs 5hp Industrial Plus, while still a flat head, it has a float type carb and a newer ,(much better) recoil design.
Keep an eye on thier web site as they get new stuff in all the time.
When serching on craigslist, you may be better off buying a whole machine just to get the engine.
Where I live, snowblowers are the best source for engines. They live an easy life, good cold, clean, air, and probably less than 20hr/year. Folks get fed up with them cause they sit for 8 months and then won't start. Every one I've torn down, was like new inside. The only problem here is that 90% are Tecumseh powered, (no help for you).
Tillers like snowblowers can be had dirt cheap for the same reasons, they sit around and then won't start when needed. While they are usually low hr machines, they tend to show MUCH more ware than snowblower engines. This is due to the hot, dusty, harsh conditions that they operate in.
You'll find lots of these with a Briggs 5 horse on them.
Third thing to look for is a leaf blower, same as the other two; low hours, and a pissed-off owner cause it won't start after sitting for 3/4ths of a year. same ware characteristics as the snowblower. Lots of them are Briggs 5hp or 8hp powered.
Hope this helps,
Good Luck