You should never have to buy a bootleg. Bootlegs should be free and available to anyone who wants them. There are a couple great blogs out there that specialize in boots, that have quite a few great Floyd ones.
Now, if you want to know what boots you should look for, these are my top 10:
1. Animal Instincts (May 9th, 1977)
2. Boston Music Hall (March 14th, 1973)
3. Cruel But Fair (April 26th, 1975)
4. Wallpower (February 28th, 1980) The last show they ever played in the US as a foursome.
5. Run Like Fuck (February 13th, 1980)
6. Keep Talking Foxboro (May 19th, 1994)
7. Interstellar Fillmore (April 29th, 1970)
8. New Mown Grass (October 17th, 1971)
9. Melbourne Soundboard (February 19th, 1988)
10. The Last Bell (October 29th, 1994) The last full concert Pink Floyd has done to date.