I want to purchase a set of TCI regional maps but at $12 a piece the set can get pricey. Anyone know of a good source for used maps? I already checked ebay and Amazon.

Powells Books in Portland and the Globe Corner books in Cambridge, MA both should have the available online.

Have you looked at a map plan showing the TCI maps of Italy? You'll see that there's a huge amount of overlap. I doubt that you'd need more than a half dozen, if that. How long are you going for? Are you really planning on biking across the entire country, including Sardinia, Sicily, the Dolomites, and Val d'Aosta? I would suggest you pick up one or two in advance for the regions you'll be starting in and for planning purposes, then buy any others you need once you're in Italy. The maps will be cheaper, for one thing, and you won't find yourself buying and carrying around unnecessary maps. I'd also suggest that maps and guidebooks are two things for which you really want the most recent editions, which used copies are unlikely to be.