I have been trying out various low clearance avoidance tools after witnessing a horrible accident and having a close call myself. Obviously I look out for the signs ahead, but not all bridges are posted and it's no good for planning a route anyway.
So what are the options?
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.
Available at: http://www.amazon.com/McNally-Deluxe-Motor-Carriers-Mcnally/dp/0528942123
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: http://www.poi-factory.com/node/1154
DiscoveryOwners.com low clearance POI database for GPS
Pros - Free to download. Accurate.
Cons - Limited data, approximately 400 entries. Many duplicates across several databases. Only available in text format.
Available at: http://www.discoveryowners.com/cginfolinks.htm
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.