1) "old school" circuits (Cadwell, Mallory and Oulton)
2) Julian's Web site for availability,
http://www.marshals.org.uk/ with that and a calendar you can organize the whole year.
3) Scanners - more clerks are accepting them. Mine packed up on Sunday at Cadwell and I felt lost without it.
4) 750MC- slick meetings, full grids (mostly) and great organizers and competitors.
5) Rockingham fighting it's way back with a dedicated bunch but always short of marshals for the infield races-come on guys (and gals) it's the British GT's in August, give it a shot.