but don't know where to
get a connection that will let me put air into the system of a gague to hook up.
It might help to find out why you are pressure testing the system. Is this code for your area?
Are you looking for a place to purchase a connection point that will allow you to add air to your system? What type of pipe did you run? PVC is not rated and is dangerous for air pressure.
If it's what I think that you are asking, you could add a tee that has a FIP thread, either add a nipple and a hose bibb and place a pressure guage on that, or add a bushing to the FIP thread and put the pressure guage into that.
If you are looking for an existing leak in the pipe, then I would just add water. It's less dangerous and you should find the leak soon enough. With air if you are looking for a leak, it only tell you that you do have a leak, not where it is.
Mick