Off the top of my head, Thomas and Betts has a partnership with Fabrimet in Canada to do lattice towers. Falcon Steel in Texas has a lattice tower division. I think Valmont has a lattice line somewhere in their large organization. Lockweld in Canada is a supplier to the USA. Comemsa in Mexico supplies lattice towers. SAE Towers in Houston has fabrication somewhere but I don't think it is in the USA but I might be wrong. There is another firm in Monterrey Mexico whose name escapes me right now.
There are several tubular pole manufacturers like T&B, Valmont, FWT, Sabre, etc that do transmission poles.
PLS-Tower is probably the leader in tower software and I maintain about 5,000 tower and pole models we use in PLS-CADD.
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I have been called "A storehouse of worthless information" many times.