Both Chas SCD and Andy ABSlotSport can supply you with lexan interiors of the 'pan' form as used in BSCRA style cars. These come with the Charlie Fitzpatrick 'Betta' shells but you can always ask for just the interiors. They are available in both 1:32 and 1:24 but with the funny 'sports scale' nature of BSCRA-type shells the actual driver sizes tend to vary so it's best to try a few and see what looks best in the car you are fitting them too.
I paint mine with a black undercoat and attach with reinforced glassfibre tape (available from both of the above).
Also available are the 'PureLite' interiors, these are a vacformed driver (a similar pose to that fitted to Scalex pan interiors), just the arms, head, shoulders and top of wheel. They are handpainted to a very nice standard and come with a sheet of tracing paper with a hand-tinted interior. Many BSCRA racers use these because the driver can be ripped off the interior and re-used in new bodies almost forever.
I also mount PureLite drivers on some black letterpaper which has the advantage of not being too obvious.
I have seen a Scalextric car at the Wolverhampton club which used a Betta pan interior with the head removed an an old Scalextric fullface helmet in it's place.
This is the Betta 1:32 RHD interior for GT/LMP cars (which as you can see is deliberately underscale. I painted this with the black undercoat technique. The whole cockpit 'top' was cut out and the tray aligned underneath and taped in.
I nicked this picture from Andy Brown-Searles site, but I'm sure he won't mind if I give this link http://www.abslotsport.com
to his store. It's a PureLite interior but a little bit small, the real thing looks much nicer