Most of the rental companies do provide some type of map, of varying quality, but there are two commercial maps which are extremely helpful to have when traveling in Belize. I use them both all the time.
One is the ITMB Traveller's Map to Belize, 1:350,000-scale. This is the best general and road map to Belize. It's US$8.95. (A company I own sells it, but it's available from some bookstores in the U.S. and at some shops in Belize.)
The other is Emory King's Driver's Guide to Beautiful Belize, a mile-by-mile guide to all the main roads and to many of the not-so-main roads in Belize. It also has fairly detailed maps of Belize City and major towns. Again, we sell it (US$14) but it's also available in Belize, usually for less than that, and Emory King also sells it via his Web site.
If you're going to do a lot of driving in Belize, I recommend you have both, but at least you should have one or the other.
Editor & Publisher
Belize First Magazine http://www.turq.com/belizefirst/
Fodor's Belize & Guatemala Guide