There used to be specific frequencies set aside for tow trucks, known as the Automobile Emergency Radio Service, but, they have been consolidated into the business/industrial side of Part 90. As well, tow companies can lease time on trunked radio systems that are open to virtually any business/industrial need.
You're going to need to search through every Part 90 non-public safety frequency in your area, and listen to every non-public safety/military/goverment trunked system in your area, unless you can do an FCC ULS lookup on the name of each tow company in your area. (And the name on the side of the truck, may not be the name in the FCC records.)
Check your state forum, and see if there is already a thread in regards to any tow truck frequencies, or start one. Other local listeners can be a valuable asset for finding and/or identifying frequencies unknown to you.
John
Peoria, AZ