$15 plus shipping,
66.102.7.104/search?q=cache:o5ZcTaFMHzoJ:www.cpwsa.com/Glock%2520Parts.htm+glock+maritime+spring+cups&hl=en&gl=us&ct=clnk&cd=2

about half way down the page.

Quoted: xcuse my ignorance, but what do these do? Cup your spring?

If the pistol is used around water (read gets wet), the slots in the striker spring retainer will allow any  trapped in the striker channel (the striker sleeve) to flow past the cups* as the striker charges forward on release.  This prevents a hydraulic effect of any trapped water from slowing the striker down the striker that could cause a light primer strike.

The down side to the maritime spring cups is that they will wear at a faster rate, and cincluding the striker sleeve, meaning the side will need to be serviced more often (cup and sleeve replacements).

*The striker spring retainer (cups) mates/slides against the striker sleeve inside the slide.

Quoted: $10 item #GLO-70M (click the pic) www.lonewolfdist.com/photos/GLO-70M.jpg

Wait, I checked the site you listed and can not find the butt plug that they sell.  Did you send us to the PG rated site?