at any hardware store
Actually these threads look like they are pipe threads which have a taper and not the straight threads.
When tightening pipe threads you would want to hand thighten them then you can use a wrench to complete the tightening of 3/4 to 1 full turn and not more. If you tighten it more then you will splay or distort the threads and they will leak.
Another caution is not to use teflon tape on a pipe thread. Since the thread is tapered what will happen is there would be excess tape that is really not fully in contact with the threads. Under pressure the loose tape can brake off within time and get pushed into the regulator or the solenoid. Then you would have equipment that you spent $$ on that is not working properly.
If after tightening to the specs. that I gave which are an "approved method" in the commercial industry and you still have leaks try to use some LocTite or another liquid or paste type of thread sealer. I just had a leak in my CO2 system I setup during the weekend and used RectorSeal 5 Pipe thread sealant which you can get at Lowes or HomeDepot.
