Read the owners’ manual to find the correct fluid and buy it at an auto parts store. Just make sure you get the correct fluid. One of the transfer cases does require special fluid (the NV247, which is used in the Quadra-Trac II and the Quadra-Drive systems).
Will the parts department sell you the fluid if you just walked up to the counter and asked for a couple quarts? You could also simply contact a different Dodge, Jeep or Chrysler dealer to see if they will sell it.
Sounds to me like you need to find a new mechanic anyways.