I don't remember ever seeing them to buy, but there are two other ways to get it. If you take the job from the czerka guys in the shadowlands, he will spray you and tach will attack you, which will allow you to take glands off of dead tach bodies. He will pay you for some, but you can make sure you save a couple for griff. The second way is there are 3 tach glands in one of the containers in that czerka camp. I believe you need security to open it.
"without evil there can be no good, so it must be good to be evil sometimes" -Satan from South Park: Bigger, Longer and Uncut
Take Mission with you, don't work for that one guy unless you want DS points. Chase them off by shuttng down the sonic emitters. Have Mission open the containrs, you get 3, go ahead and use Mission to disable the other mines on the level for sale or later use. I sell them. Get the droid head and go back up, settle the merchant quest and switch Missin out if you want, I put Juhani in. Go back down and you'll easily finish off the remaining wolflike creatures if you wish, and pick up a few average items. Then talk to Jolee. A total wuss of a Jedi, but you have to take him. Mission is tougher than this guy. Serious. At lvl. 20 she has about 20 more hp and with twin ungraded 'Cassus Fett' blasters can do more damge than he can. When you leave the planet you can take the glands to Mission's brother.