Otari or Technics Reel-to-Reel ?

There are lots of sources for blank tape. I use RMGI, bought a couple cases of it when I got my deck and have purchased more since. The better question is where to find a good deal on blank tape. It is not cheap.

As for pre-recorded, again sources exist, depends on what you are looking for and what you want to pay. 7 1/2 ips is more accessible and more titles. In the OPs case though I do not think pre-recorded tapes were what he was looking for. More so to do home recording.

You can find a lot of blank tape on ebay. Seems to me ATR is very much in production- a lot of studios are still using analog tape, enough to keep tape manufacturers alive.

Orpheus10, you may be right about 15ips for records. I will listen, if I hear no difference I'll use 7.5ips. Depends also on the source. My Nottingham Spacedeck/Spacearm is good but far from top of the line, the same with phono stage.
Yes, I would be interested in finding pre-recorded tapes but also finding the recordings that were never released. You see a lot of this on youtube. There are recordings of great performances, someone has the master tapes and copies. Difficult to find, you need to be a member of audio clubs and have connections, but I think with time and patience it can be done. As for the cost, well, certain things are worth paying for. Besides, I am very selective in what I listen to. As an example, out of probably one hundred concerts of Mahavishnu Orchestra that I found on youtube I listen to two. The same with Miles Davis.
So it would be an interesting project.

There are a total of 8,000-10,000 pre-recorded tapes offered every day on auction or buy-it-now basis on eBay. Prices are all over the map and most of the tapes are estate-sale purchases, offered by sellers who have no decks on which to play them. It's pretty wild but you can get lucky. I just won a classical album of Handel works by E. Power Biggs for a grand total of $5.04 -- $1.04 for the tape and $4.00 for shipping.

When you search for blank tape on ebay, use the key word "NOS" after "reel to reel tape." This will search for tapes in sealed boxes and "one pass," or lightly used tape.

Mind you, the selection ranges from older poor quality tape to high quality mastering tape. You should familiarize yourself with the different tape stock if going into this hobby.