Where to get the mag block inside G17 mags?

I have a ton of G17 and G15 mags and guts, and I realized the neutered 15 rd G17 mags just use a G15 spring with a little piece of plastic to block the extra space. Tried searching online but don't know if I'm able to just buy those little plastic stopper pieces so I could make my own 15 rounders I could carry.