Try
Coast Pneumatics. Just type the item number from the SMC solenoid into the part search page. Last time I checked (about a month ago), they had a bunch of the solenoids identical to the SMC ones from the Kit of Parts of seasons past, just not necessarily in stock.
Single Acting:
VQZ2121-6LO-N7T
Double Acting:
VQZ2221-6LO-N7T