Well I've used a fair bit of waterslide paper in the past so feel qualified to give input here. It makes for a very thin decal that's a lot easier to bury underneath automotive clear than vinyl, which I what I primarily use it for in my hydrographics business.
You basically have 2 options to choose from laser printer or inkjet, I'd suggest whichever you happen to already have as then both work fine with one caveat - if your inkjet is dye-based ink it'll likely bleed unless you clear coat (rattle can lacquer works great) it before dunking it in the water to apply. If you're using an epson or something else with pigment inks (or laser toner) this isn't a problem as the pigment inks are not water soluble.
They both will come with either white backing or clear backing, and either of them can be contour cut with your plotter. You will want to leave a small amount of bleed. Be aware they are very thick to print (heavier than card stock) and thin to cut so plan on a little bit of scrap to try to get your settings dialed in just right. If laser, use the heaviest paper setting possible and run a sheet or two of regular paper right ahead of it to preheat the fuser for the best quality.
If you ARE planning to clear coat over either of them (like I did with automotive clear), a light mist of the lacquer prior to your regular clear is helpful for the laser, the toner can bleed just enough to give fuzzy edges if you hammer the clear on too heavy to try to bury it faster. Only ran into that when I was in a hurry though of course . Never experienced that with the epson printed inkjet one.
Hayes paper is one brand that I know of. I'm pretty sure that's what this gal sells in small quantity if that's what you're looking for, and I've had good experience purchasing from her in the past, primary though eBay and then her website:
https://www.etsy.com/listing/196599278/waterslide-decal-paper-white-clear-for?