Howdy,
Sliders are available everywhere online. I got my from scooterworks, & used 18x14 13 G Dr. Pulley's. If I were to do it again I'd probably go a hair lighter, but the 13's worked, & I live in hilly terrain. They did a good job of killing the flat spot in acceleration, but you won't improve top end with them particularly. Scooterworks was a bit meh to deal with, my order was missing a piece, & their prices (especially on the belt) were a bit high. On the plus side they did ship and get (most of) the stuff there when nobody else would. So they ain't awful either.
Tires, if you're running OEM, even if they look good I'd dump 'em. Michelin City Grips were great on the bike. They found grip in literal floods. Not 'nuff to do fancy maneuvers in the flood but held 'nuff to keep it upright and moving when it was looking ugly. Great in the dry too, I run 'em on my HD as well.
Plastic - I went the the "kymcopartswarehouse" (powersportspartswarehouse) and had good luck. Bear in mind this stuff has to be specially shipped so can take a while sometimes.
Beyond that AF1racing has some stuff, & of course your local dealer should be able to get parts too. There are other scoot shops but those are the ones I've dealt with.
Cheers
Tromper
