MyFont's web utility WhatTheFont? takes an interesting approach to selling type: If you want to use a font that you've seen in another publication but can't identify, you can upload a screenshot, scan, or URL (GIF, JPG, etc.) of a bit of text and MyFont will try to match it to one of the fonts they sell.

I can't vouch for the quality of the fonts, but I uploaded some samples and was able to find relatively close matches for around $30 apiece. Not as good, obviously, as locating the actual font you're attempting to duplicate, but a useful option in some circumstances, like when that clients gives you a copy of a magazine ad and says, "I want the text to look like this.