Ebay usually has em. You can test them by applying power from a 9v battery and seeing which one pulls a vacuum.

 

@Tominlaw
I assume the person means they buy used off ebay, not new. The 9v trick kinda works, they are a simple "switch" but for vacuum. If when not powered you can't blow through, when it's powered you should be able to, or the exact opposite. It's a way to electronically control which circuits have vacuum. On the electrical side of things, they are similar to a relay. For 4x4, there should be one to engage the 4x4, and one to disengage it, atleast that's how my 98 T100 was setup. I ended up bypassing them completely and ran the vacuum directly to the front diff on the engage side and installed manual locking hubs from a 86-88 Toyota pickup (same as 96-89 4runner too).