I think you have to actively collect and then "cherry pick" good typefaces from larger collections.
CorelDRAW is probably the best starting point for adding something alonside your dedicated sign making program, even if you don't use Corel. The 1000 fonts bundled are high quality and from vendors like Bitstream, ITC and a few others.
Adobe's Creative Suite 2 Premium and Production Studio packages don't have nearly as many fonts as Corel. But the 200 or so OpenType fonts bundled are top quality. Some of these fonts, such as Garamond Premiere Pro, have some additional capabilities that are unlocked in IllustratorCS2 and InDesignCS2.
Outside of that, you have to look for good fonts where you can. Some of them you'll have to buy outright, but incorporate into the cost of the job (not too hard to do if you're working on a larger permanent sign project). You'll find others in books about fonts, graphics magazines like Computer Arts, etc.
Speaking of OpenType, some of the fonts sold at Letterhead fonts are now including special OpenType ligatures. House Industries also has some excellent quality OpenType fonts. Their $169 Ed Benguiat collect includes a display face called "Ed Interlock" that has an exhaustive number of special ligatures which can be activated in Illustrator.
