I believe it is Rennlist that sells replacement wheels - they are a hard plastic/nylon. they also pop on (and off pretty easily) - others on this forum have recommended getting a good used wheel rather than using these. They didn't work for me although I had the regulator bind up and shift the window channel forward about an inch on the door glass and also chewed up the wheel and put a bow in the actuating arm. I got a used regulator with good wheels instead. You don't say which year your car is so... the later years (mine is a 91 s2) have a more robust regulator than early years - the casting is heavier - but otherwise identical - motor, mounting points, window travel, etc.
Once you get your wheel back on - you need to figure out why it came off. Check your window glass carefully for signs of the channel having shifted - you should be able to see slight marks where the sealant let go if the channel has shifted. If not - when you get it back together check your window through full operation to see if the wheels are going to come out of the tracks. If - like mine - your channel has shifted Porsche will gladly sell you a new window with the channel attached - these are not available separately as they were for older 911s. My fix was to carefully cut the old adhesive (being careful to NOT cut the rubber that lines the channel), clean all the old adhesive out of the channel, the rubber and the window. Mark the correct position of the channel on the window glass (I had to open up my other door to measure the placement) and then take it to a windshield shop to have them glue it all back together with modern adhesive. The windshield shop was sure it wouldn't work so I had to agree that I wouldn't hold them accountable if it came apart when I tried to use it. That was a year ago and so far its no problems.
