Where can I find T&B (Thomas & Betts) circuit breakers, or compatible?

The breaker you show is not a double pole breaker. It is two single pole breakers in a single  1" space, often called a twin breaker.  They are made in 2-15 amp, 15-20 amp, 2-20 amp, 15-30amp, 20-30 amp and 2-30 amp and others.  If you have two wires hooked up to the breaker, you have two separate 120 volt circuits. Be careful what you do, you may cause a fire, or worse zap yourself.  Former electrical contractor.