If it is the distributor O-ring, then it is an easy fix; just be sure when you take out the distributor, you mark the angle of the flange so when you put it back in to the valve cover, you won't have to re-adjust the timing too much.
If it is an internal leak to the distributor, then consider getting an internal distributor rebuild kit .
www.kbox.ca Kbox Engineering makes distributor rebuild kits that also come with the distributor O-ring. These kits are meant to rebuild the distributor internally; I just used their 4AFE kit to rebuilt my distributor.
You may also want to get a rebuilt distributor from a place like eBay. Getting a new distributor from the dealer is costly, and I would not recommend a junk yard if you need internal gaskets and seals; these are often the first to go on these engines anyway.