Used Nikon Sb-28 or Sb-26, or a new
Vivitar 285HV
That and a few hours to spend reading 'Strobist 101' on strobist.com to bone up on how to use off-camera manual (non-ttl) strobes creatively and effectively with minimal gear.
I'll admit fully that I've got zero interest in flash automation when it comes to exposure, but with $100, some spare time, and a bit of practice, you can get killer results out of any reasonably powered manually-controllable flash (ie. one that lets you manually control the flash output).