For a 75 gallon aquarium, I would recommend that you not use a diffuser, but an inline reactor. It will be much more efficient in diffusing your CO2. You should be able to find instructions for DIY-ing your own inline reactor online.
For the solenoid, check valve and bubble counter, eBay or the Swap and Shop is likely the best option. There are definitely some North American vendors for the solenoid, as well as check valves (I assume you are looking for brass ones and not plastic).
For the bubble counter, depending on what style you want, your options may be limited. I believe the JBJ styled bubble counter is now only sold in Asia, so you may have no other option but to purchase from Asia.
For the solenoid, check valve and bubble counter, eBay or the Swap and Shop is likely the best option. There are definitely some North American vendors for the solenoid, as well as check valves (I assume you are looking for brass ones and not plastic).
For the bubble counter, depending on what style you want, your options may be limited. I believe the JBJ styled bubble counter is now only sold in Asia, so you may have no other option but to purchase from Asia.
