The rubber plugs with the 'tire stem' shouldn't be hard to find. However, who needs them?

Down in Va. when we'd pressure test lines, we used an air compressor, you could rig up something like we did. Using a properly sized male adapter, ~ 1' of schedule 40 PVC pipe and a glue on cap, you can drill through the cap and use a tap to install a 'tank valve' (tire stem with threads on the bottom).

This would functionally replace the rubber plug with the tank valve through it and let you blow out the entire system from one skimmer (with the multiport on 'recirc'). It will cost more than the rubber plug (you need the M/A, pipe, cap and glue and primer as well as the tank valve and the tap) - but it's very simple to throw together and will last the life of your pool.

If you still can't find the blow through plug and need any info on building what I just told you about, just give a holler and I'll help you build one