lets get this perfectly clear....
what marker do you have??????
if its a 'low-end' semi, eg spyder, pirranha (sp?), the mechanical versions etc, then you have to buy a whole e-trigger frame, which will replace your mechanical one, there are limited numbers of custom trigger designs available for these guns, but you still need the trigger frame in the first place.......
if its a 'high-end' electro pneumatic, eg impulse, timmaaay, etc, then yes, you can can buy individual triggers as the electronics are already present, however you need to check that it is compatible with the board and frame you're using as different triggers for the same gun only work on certain circuit boards. and the same for frames, an i-frame trigger wont fit in a ordinary .45 frame.
but still you can get differing frames, as i said above, eg i-frame and werm-frames both fit an impulse, but you could not simply swap the triggers.
you were being a bit vague...... we need more info mate!
Hp