The trick is the NPT threading....
Aquatic Ecosystems has them... lemme just whip out my trusty catalog (which is a MUST have)...
http://www.aquaticeco.com">www.aquaticeco.com</a>. Part Number: BNT4
If you use the search on their web site it comes up as a 1/8" tap, but if you click on it it shows them all and you can select the 1/2.