I think what you want is an ISO1050:
It's an isolated transceiver. One side connects to the processor CANTX and CANRX, and the other side connects to the bus CANH and CANL. Two isolation voltage levels available. Only surface-mount, no through-hole available, but the 8-pin variant is a DIP8 with bent legs, so it's easy enough to solder by hand.
Of course, it sounds like you just want to isolate the bus itself, and not feed data to a processor. But I think you should be able to put two of these devices back-to-back. Tie the CANTX from each to the CANRX on the other. I haven't tried it, but with appropriate power supplies, it sounds like it should work.