I get them at Tessco. Should be about $35 for one without a mount, which is the STI-CO part number ROOF-FT-NITI-M.
They are $60 if you get the ones with a mag mount or that include an NMO mount. What your are looking for is the replacement whip option, no connector, no mount. Well, unless you need a mount or NMO, in which case the replacement whip is not the right option.
As I recall they just come in a plastic bag with a rubber seal and not much else. They are targeted for commercial installations and I suspect they buy them by the case, so they don't need instructions that will just clutter up the shop.