Low-Tech Companion has some survival rules. Also political and economic rules if your post-apoc world is thrown back far enough for that.
There's a fan-made GURPS Fallout that has some scavenging tables.
There was a post-apoc book or series in development that was
rumored last year, "After the End" as the working title. Last seen on "
indefinite hold", but I don't recall any recent status.