Which external flash units work with Alpha cameras? Recommendations please.

I think any Minolta flash released since year 2000 is ok (see this list http://www.mhohner.de/sony-minolta/flashes.php )

Generally, you want a flash unit with a mechanical power switch. Electronic switch in 3600HS(D) aka HVL-F36AM drains batteries, so you have to remove batteries if you don't plan to use flash for several days.

Also with older flashes you lose ratio selection in wireless mode (unless it's A900 body, don't know about A99, but these will requre top external flash to drive wirless flashes). Newer flash models have ability to select ratio in the flash unit itself.

Interestingly, but I don't like to use at home more powerful HVL-F58AM unit, I prefer 3600HS(D). Both on A65 and on A700 bodies.