The SB-20 is a decent flash. Owned one for a while. Only issue I came across, is that since the bounce head was enclosed in the film canister shaped lens at the top, you couldn't easily attach a diffuser to the flash. The 22 is similar, a little more modern, with an exposed bounce head, but still not compatible with standard diffusers. There is also the SB-16B, which is a workhorse with almost the same power output as an SB-24. Does have the secondary flash tube. The 20 and 22 usually seem to go for $30-$50. SB-16B's, $20-$40. SB-15's, $15-$30, depending on condition. I have seen some SB-24's in the $50 range recently, so they aren't terribly expensive. Also, if you have an off-topic Nikon body with an SB-600 or 800, one of those flashes will work nicely on your N8008s.
-J