Brownels is usually out.
Amazon and ebay can be hit or miss. One ebay action had 50lb's of taps and dies from a closed business and I missed the auction by 10 bucks, but I only put my max in and if I get the auction or not I don't bid more, stops me from spending to much.
Enco - has them in lots of differing grades.
1/2-28
ht tp://www.use-enco.com/CGI/INSRIT?PARTPG=INSRAR2&PMAKA=891-4838&PMPXNO=3904728
5/8-24
ht tp://www.use-enco.com/CGI/INSRIT?PARTPG=INSRAR2&PMAKA=314-1866&PMPXNO=2619616
Most any full industrial machine supplier will have them, if your close to a big city may be as easy as opening the phone book and going for a quick drive.