All the original recordings, re-recordings, and a DVD of a
concert in Oslo are available.
There are even some cheap (and badly transferred) "public domain"
reissues from the UK.
Below are links to items available at Amazon, but shop around
for better prices.

Best wishes, Thomas.

http://www.amazon.com/The-Tom-Lehrer-Collection/dp/tracks/B0039TD73G/ref=dp_tracks_all_1#disc_1

http://www.amazon.com/Remains-Tom-Lehrer/dp/B00004SWBH/ref=sr_1_4?s=music&ie=UTF8&qid=1365353941&sr=1-4&keywords=tom+lehrer

http://www.amazon.com/s/ref=nb_sb_sabc?url=search-alias%3Dpopular&pageMinusResults=1&suo=1365354145250#/ref=nb_sb_noss_1?url=search-alias%3Dpopular&field-keywords=tom%20lehrer&sprefix=tom+l%2Cpopular&rh=i%3Apopular%2Ck%3Atom%20lehrer