Not sure whether you mean you can't find an original tape for sale - it's probably the one that people are least likely to sell, but from what you have written earlier you may already have some material found in .TZX or .P file format so I assume you have a way of converting those to audio playback format.
Have a look around on this site, where there is a lot of useful archived material including books, various utilities, and other things.
http://www.zx81stuff.org.uk/
For example, here:-
http://www.zx81stuff.org.uk/zx81/tape/3DMonsterMaze(NGS)
I personally also had great fun with 'Seawolf', which is a kind of early forerunner of 'Silent Service', the submarine simulation.
http://www.zx81stuff.org.uk/zx81/tape/SeaWolf(Newer)