You can actually pick up the bridge covers for $15-$19 from
http://guitarpartsresource.com/bhardware_ashtrays.htm in chrome, gold, or black (bottom row.) They're sold as EB bridge covers, but they're the same size. Aside from the very early EB0s which had a narrower bridge cover, to my knowledge all Gibson basses use the same bridge cover. The neck pickup cover was only used on Thunderbirds, which is why you never see them. Some people tossed them out, other people kept them on their basses (and won't be taking them off any time soon.) Meanwhile, the bridge covers are being mass produced by a few different groups of people.
I asked a guy on another forum where he got his, as it looked different than the real one so I figured it was a repro. He told me that he's using a P-Bass pickup cover for the neck. Apparently the hole spacing and overall width of the cover is the same as the original Gibson cover, but the height is shorter, so when you use it with the EB bridge cover it sits noticeably closer to the strings than it should. I guess that wouldn't look too bad from the audience viewpoint on stage.
I've been in the market for a neck pickup cover since around July. I saw one turn up on ebay about a month ago, bid on it, and then the auction was taken down. I sent a PM to ebay user klmahlke about the neck pickup covers and he said that he used to mass produce those and sell them for $65 but there was such a small market that he doesn't make them anymore. I really think someone should make a small batch of these, I know a fair amount of us would buy them. If I don't see one turn up soon, I'll probably go with the P-Bass cover and more than likely regret not waiting.