I agree with Stoutblock and others: I'm not sure what some were saying as many prerecorded tapes were put out in both 3-3/4 and 7-1/2 IPS. Most of the works from the Beatles, for example. As mentioned, the ratio towards higher speed becomes greater in the Jazz and Classical genre, but many prerecorded Rock tapes were released in 7.5 IPS. It seems that, like today, pop was released in the cheapest format (lowest speed) possible. I would, however, argue against the notion that all 7.5 IPS tapes are superior to the 3-3/4 variants. While typically true, I've noticed many exceptions. It depends on many factors, including tape formulation and the playback deck, YMMV. If I want a particular album, I just go with what was released. If the tape is good, it typically outperforms the vinyl and cassette versions. If I REALLY love the album, it may be available in 2-track 15 IPS from various modern production houses. This comes at a price, as the OP stated, but can be worth it as there is nothing better, save the original master (I assume).
To the OP: Ebay, Discogs, Goodwill, and even Bartertown here, are the best sources for used (and sometimes NOS) pre-recorded as the days of randomly running into 7" tapes for cheap at your local record store are few and far between and no modern houses are currently producing them (there was a campaign started on this forum to get Analog Productions to make 7" releases but it was in vain as it doesn't pencil it out). I've heard of legendary finds on Craigslist, but I've never witnessed any. What, specifically, are you looking for?