It will most likely be the seal on the front of the trans that the torque converter slips into. This seal should be changed every tine the trans or converter is removed.
To replace it you will need to drop the trans down so you can remove the torque converter. The seal is then exposed and it is a simple matter or prying out the old seal and tapping in a new one.
Usually if the transmission is leaking from the front the seal is worn out.
The other posibilities are worn converter hub, worn pump bushing (these two usually will go together ) transmission pump gasket and o ring.
Check 'em all.