I bought several Teflon sheets on Amazon - they're supposed to go in the bottom of your oven to catch drips so they're black (not very pretty) but they're more than twice the size of the largest Super Slider and cost about 1/8th as much. I bought four and used Gorilla tape (also black and fairly slippery) to tape them all down so now almost my entire sewing surface is slippy. It helps a lot - I leave them on all the time. I cut one so it fits around the plate on the bed of my machine, so the plate is still removable for cleaning out the bobbin without me having to remove the Teflon.
I've heard of silicone spray being used but never tried it myself.