Rand McNally Motor Carrier Road Atlas
Pros - Much more than a low clearance list, with an abundance of useful information.
Cons - Limited to just over 1000 low clearances. Can't use while driving. Costs around $50.
POI-Factory.com low clearance POI database for GPS
Pros - Free to download. Accurate. Ready to install.
Cons - Limited data, not much more than 300 entries.
Available at:
Low Clearance Underpasses | POI Factory
LowClearances.com low clearance POI database for GPS
Pros - Huge database, up to 10 times larger than the others available. Accurate. Regularly updated. Ready to install.
Cons - Costs $20 for annual subscription.
Available at:
Low Clearance GPS Data
There are also a few fully integrated GPS systems available, but these are very expensive and I cannot vouch for their accuracy or the number of low clearances in their databases.
Hope this is of use.
Gringophil