There is a man doing business in my county - building large metal buildings. He takes several thousand as a down payment and then quits working. Or he does all of the work but fails to pay subcontractors even though he has been paid the full price of the building. He has done this to over 20 individuals. Many of these victims are from outside my county. I am not sure I can prosecute the cases that occurred in other counties. The contracts for the building and the money exchanges hands in the county where the construction is to occur. Anyone have any thoughts on this? I would appreciate any assistance.

Take a look at State v. Weaver 982 s.w.2d 892 (Tex.Crim.App. 1998). It discusses aggravated theft in multiple counties. It is cited by Diane Beckham in the TDCAA Criminal Laws of Texas

Check out Chapter 162 of the Property Code. This situation appears to be misapplication of trust funds. In terms of venue, it would be the county where the misapplication occurred. Where the building was constructed or money received would not control for venue purposes.