If you're in the UK (or maybe the rest of Europe)...
http://www.legendgames.co.uk/acatalog/GURPS_Traveller.html
I bought the bundle of four alien books for GBP 16.00, which is shockingly cheap for SJG stuff.
http://www.legendgames.co.uk/acatalog/GURPS_Traveller.html
I bought the bundle of four alien books for GBP 16.00, which is shockingly cheap for SJG stuff.