Hi Rhapsode,
I had the same problem a few months ago when doing a complete clean-out of my water tank, & refurbishing the 2 access hatches.
I discovered that you can't actually buy ready-made 'O'-rings for these, so I Googled 'O'-ring manufacturers in Cardiff', & 1x hour later I'd found the place (sorry I don't have the name & address to hand) & they measured the diameter of the 'O'-ring groove & made me 2x 3mm & 2x 4mm 'O'-rings. You can also buy lengths of the material & cut to suit anything you need, & just Superglue the cut ends together like they do.
Mine cost about £3.00 per pair.
Jock
Sorry to dredge an old thread...
The link above is dead but if you do a search for Service kit AK4010 which fits the Whale TCL2 and TCL4 covers lots of the chandleries do them