Protor Teagan is glitching out and won't talk to me (similar to the Esbern glitch in Skyrim) so I can't buy the signals from him, is there another way to get them?
If you complete Rockets Red Glare or otherwise beat the game in a manner that sides against the Brotherhood, the faction you choose will begin to sell and fly Vertibirds. For example, if you destroy the Prydwen for the RR, Tinker Tom will become your new source for the signal grenades.
Please note that it's possible to make choices that give you no access to the Vertibirds going forward, but that effectively requires you turning against everyone and having no friends. This is possible using the Nuka World DLC.