it seems that unless you are looking for something specific or an auction, there are no definitive sources... to satisfy my own curiousity, i poked around a bit and came up with a few links...
an incomplete list of quad reel to reel releases --
http://members.cox.net/surround/quaddisc/quadQRP.htm
and...
the beatles on reel to reel --
http://www.friktech.com/btls/tapes/rtr.htm
virtually any album released from the early 1950s until the early 1980s could have been available in this format, but the way the companies approached the technology was rather cautious at first and totally schizoid by the end...
early on, an unwavering focus on the hi-fi market meant that jazz, opera and classical recordings dominated... rhythm & blues, gospel, country and rock-n-roll was not issued at a time when all of those genres were reaching a new peak... but, then again, many of the best artists of the time were on little shoestring labels like sun, chess and king and not the sort of stuff that anyone in the big machine wanted to acknowledge...
the first stereo recordings were available on tape before they debuted on vinyl... this new medium opened the door for more releases like musicals, movie soundtracks and modern sounds from composer-arrangers like henry mancini, esquivel, martin denny and others... however, very few country, teenage-pop or rock-n-roll records were released on reels until the mid 1960s... but when they did, some real gems came out... to my ears, virtually anything from atlantic, motown or stax is very delicious on reel to reel...
economics were also a factor, so many independant or smaller companies never made their catalog available on reel to reel... in fact, from what i have seen, reel to reel was a game played by the big boys and only truly widespread from top to bottom in the industry for a handful of years in the early to mid 1970s... this was perhaps due to the rise of quad and the dominance of another 1/4" tape based format, the 8-track, but that is purely conjecture on my part...
i sure wish vagabond records was still up there in the 'Couv' -- remember that place downtown? back in the day, they had a once a year sale and every title in the store was a dollar... you could only buy 20 a day, but it was plenty to make you happy! we'd jump the old 5 bus from portland, buy 19 rare titles we needed -- mothers of invention, united states of america, phil ochs, syd barrett, the who, etc. -- and one gem to sell when we got back to town... more often than not we broke even... they had a mess of reels too... these days, if you are looking for reels, a few places in portland are worth a trip... mississippi records, crossroads music -- while you are there, talk to erik about reel to reels! -- and everyday music all have at least a few good ones any given day...
good luck!
dave