Where can I purchase an aftermarket air bypass valve?

About the only place I have seen them is at a dealer or a salvage yard once you get back pre 2000 or so. I believe yours has the Ford EITE-9B289-HA valve with the hoses on both sides and a vacuum port? Like the image? If so you might find one on eBay or Amazon. Ford used the same basic 9B289 valve design with different mounting and hose options but they all work the same. Even the later all in one designs work if they don't have the electronic control valve on the. If yours is sticking you may want to check the check valve on the tube to the cat. They can get pinholes and send exhaust gas into the valves and cause problems.

If that is the valve you have someone looks to have replaced it with a newer version already, that looks like one off of a 5.0 or the 5.8. Those are available aftermarket. As for bypassing the system, I wouldn't unless it was a very last resort. The way the system works is to use the TAD to divert air into the manifold at startup to counter the rich mix from the ECM until it goes closed loop. The extra air helps the converter to ignite faster with the extra fuel and get the front O2 up to temperature quicker. Then once the system switches to closed loop the air is diverted so it flows to the cat most of the time, but on any deceleration the TAB then dumps the air into the atmosphere because the system goes very lean at that point and the extra air then could cause the metals in the cat to overheat and damage them.

The high idle is a vacuum leak in the diaphragm. Common failure.

Now if you live in an area that doesn't do ant testing or visual inspection, you could just delete the entire mess. Block off the ports in the manifold and the air tube that goes to the converter and remove the pump, it will run OK without it, but initial start up in cold weather might be a bit rough until it goes closed loop.