The SB-900 is definitely not a bad buy any time, even if you’re just starting out. 🙂
You can get a used (Like New) one from Adorama. (Just scroll down to “Shop Used” and select from the dropdown list.)
If you want double the used warranty (180 vs. 90 days), check out KEH’s listings and find the price and condition you want to pay. 🙂
If you don’t need such a fancy flash with all the bells and whistles, just search KEH for SB800 or SB700. They get progressively cheaper because they’re older, and they all do pretty much the same thing. Some are more powerful and recycle faster than others, Ken Rockwell’s chart will tell you all the info so you can make an informed decision.
Any of these officialy Nikon flashes, (SB700, SB800, SB900) should theoretically be better than the cheap Chinese clones, so I don’t think you can go wrong with any genuine Nikon branded flash. 🙂