Chris,
anything like this you want to buy from the States, (where these things seem to be found in skips, or garage sales for $5!!!). Next best place is probably Germany. First, they are hard to come by on these shores, and second, if you do manage to find one in reasonable nick, they are likely to blow up! (hehe....reputable dealers don't have many - plus you pay a fair whack, and private sales require a LOT of precaution. I'd definitely demo if possible)
The sequerra is probably difficult to find, and is failry pricey by all accounts (couple of grand?)
The Revox (B760) is probably not that difficult to get hold of even in the UK. (In fact IIRC i think Simon Connell has one) I haven't followed the market for a while, but they used to crop up fairly regulalry (possibly becuase they were used in studios?). A decent one would probably be <£500.
The Marantz would probably cost more, as they carry a bit of kudos for whatever reason, and prices fluctuate dramatically.
A lot depends on condition and service history with these things.
TBH, i would personally pay over the odds if i was assured of getting a good 'un.
If you were serious, i expect Audiogon would be a good stop. And if you were going to spend decent wedge, try and get something which has been renovated professionaly (again - difficult to get done PROPERLY in this country - and i'm not just talking re-alignment - i know of some places in London, ut then i've never really got that involved that much in these things so i'm only an amateur). Ebay in the States has stuff in abundance, but take the normal precautions.
Here's an example of a nice Sansui piece that has had a proper going over (obviously its an amp which involves a lot more, but gives you an ideas of a 'proper job':
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3049313215
That's not meant to put you off, just air on the side of caution, even though its only a tuner. You may be lucky at the end of the day with a bargain find.
look here for a guide on tunas:
http://www.fmtunerinfo.com/index.html
cheers