I'm just looking for cheap (<$15) RP-SMA omnis. I've gotten them by buying super-cheap wifi cards before, but that still seems like a bit of a waste.
It's true you could build them yourself, but rubber duckies are an example of modern cheaply mass-produced parts, that are made in huge volume. That is, there's no point in trying to produce it yourself, you can't come anywhere close to their price.