Of course, by no means all recordings of the mighty 'Tron are officially available. The bootleg market holds several delights for the Mellotron aficionado, although King Crimson seem to be doing their level best to release almost every live recording of the band ever made...
Here's a short list of Mellotron-related recordings that you can't pick up at Our Price (practically all of 'em, then...).
Picnic at Hanging Rock
Two tracks from this are available on an album entitled 'The Best of Australian Films' (or similar), but no official release for the excellent discordant Mellotron choir on the soundtrack.
This Life & Times of Grizzly Adams
Not to be confused with the later TV series, this fairly low-budget film's budget obviously didn't run to an orchestra, so the bulk of the soundtrack is played on a Mellotron. And very nice it is, too; several excellent themes, including a very impressive 'Indian falls down a cliff' sequence. No, really.
The Martian Chronicles
Chris Haley tells me there's a load of 'Tron choir and strings on the 1980 TV film of Bradbury's classic; I shall now attempt to track a copy down...
Gerry Anderson's second non-puppet series (after UFO), from the mid-'70s. Again, haven't heard them for myself, but apparently a good deal of 'Tron choir on the soundtrack.
There are inumerable bootlegs of Beatles studio sessions that have been doing the rounds since the early '70s. Several of these document the Strawberry Fields sessions, including the band's various Mellotron experiments - one version has the intro played on brass, not flutes. Very odd...
Various live stuff, '72-'78. Despite the long overdue release of the 'Archive' boxes, there are several relevant Genesis songs only officially available in studio form, including 'Can-Utility & the Coastliners', 'Seven Stones', 'Eleventh Earl of Mar' and 'White Mountain' from the '76 tour.
Despite Robert Fripp's attempts to release every note ever played by Crimso, there are many relevant boots, especially from '73-4 containing unreleased performances. Of particular interest are ones featuring the band's unique improvs.
'Kashmir' live is the obvious one here. Until Zeppelin put out a live box or something, track down almost any boot from the '75 or '77 tours to hear it in all its on-stage glory.
Although Yes' '70s career is covered by a triple and a double live album, there are several tours under-represented by live recordings. Any boots from the 'Tales' tour (1973) or 'Relayer' (1974) will feature much 'Tron played by Rick Wakeman and Patrick Moraz respectively. The much-circulated BBC tape from Wembley '78 has Wakeman playing Birotron.
Gentle Giant only used Mellotron for two or three years, as Kerry Minnear apparently found it 'too unstable' for touring (haven't we heard this somewhere before?). There is, however, 'Tron to be heard on a few boots, including 'Glass House Tour'.
Pink Floyd never used their Mark II live, but it can be heard on a few BBC session tracks from the late '60s.
'Forgotten Songs', a boot containing early rehearsal performances has some 'Tron choir action. Can't see an official release for this one, somehow...
1980s UK Prog
Pallas and IQ both used Mellotron on stage for some years, with Pallas carrying on until 1988. There are various tapes in circulation featuring tracks never released in live form. Twelfth Night used 'Tron early on; early live tapes contain unreleased tracks which may contain hints of 'Tronism. Then again, their Mellotron use was never exactly innovative... A couple of smaller bands on the scene used 'Tron too. Anyone got any live Tamarisk tapes?
And the less obvious...
Bee Gee Maurice Gibb's unreleased solo album from 1970, provisionally titled The Loner is supposed to contain a few 'Tron tracks. From Sweden, Kaipa's 'Stockholm Symphonie' appears to be a radio recording from the mid-'70s; Änglagård and Anekdoten also have live tapes in circulation. ZZ Top (!) are rumoured to have used a foot-controlled Chamberlin (a remote, presumably) on their 1977 (?) tour, mostly to replicate sax parts - tapes are in circulation. Various 1974/5 Hawkwind boots feature 'Tron - an official release from 1974 covers one of the two relevant tours. There's also a fantastic Schicke Führs Fröhling (SFF) live double-disc set from '75 with more 'Tron than you can shake a stick at. There are doubtless many other lesser-known Mellotron users whose gigs are traded around the bootleg circuit - all information gratefully received!