TEA lasers are basically just a few pieces of metal and high-voltage, seen them at high-school level quite a few times: http://www.sparkbangbuzz.com/tealaser/tealaser7.htm
CO2 lasers are relatively complex and are easiest if you buy some parts readymade, but can be completely homebuild as well: http://jarrodkinsey.org/co2laser/co2laser.html
There are tons of ressources online, a good overviw over different possibilities and basic information can be found here: http://www.repairfaq.org/sam/laserfaq.htm#faqtoc
TEA lasers are basically just a few pieces of metal and high-voltage, seen them at high-school level quite a few times: http://www.sparkbangbuzz.com/tealaser/tealaser7.htm
CO2 lasers are relatively complex and are easiest if you buy some parts readymade, but can be completely homebuild as well: http://jarrodkinsey.org/co2laser/co2laser.html
There are tons of ressources online, a good overviw over different possibilities and basic information can be found here: http://www.repairfaq.org/sam/laserfaq.htm#faqtoc