I am not a big fan of flash photography, but there are times when indoor shots need the light so it is a necessity. The camera comes with a very small built in flash that you pop up to use, but it is pretty much useless - the lens always gets in the way for any kind of even lighting.
Sony carries their own models for the A100. They are pretty much rebadged models of the Minolta Digital flashes. The new Sonys are quite pricey. The used Minolta models are also quite pricey - if you can even find them. They seem to be at a premium these days.
There are a couple of non-digital older Minolta flash guns made in the 1990's. From what I have been reading, they can be fiddled with to work somewhat with the new DSLRS. Many of the functions will not work, and you need to calculate everything manually to get the best results.
There are several third party brands available, but Sony/Minolta doesn't take the standard hot shoe that every DSLR and SLR on the market takes. They do make adapters to fit the flash gun to the camera, but I have been reading about the volts that the flashes deliver through the camera, and the last thing I want to do is blow up the camera's electrical system.
I have not decided which gun I will go with yet, the newer or the older one. Considering I will not be using it overly much, I am sure I could get by with the older, non-digital one. Now to try to find one of those - they are also hard to find and overpriced.