Unless things have changed very very recently, you will have to hope that someone who has previously purchased the mould(s) you are looking for is willing to part with it or you will have to get in on a group purchase or you will have to custom order from Lee.
Custom orders from Lee carry a $100 set up charge on top of the $50 mould price (6 cavity). They will wave the set up charge if 25 or more moulds are ordered. So, you either need to get in on an existing group order or find enough other people interested to order the required minimum. Of course, if you are so inclined, you can special order just for yourself. I was very fortunate to get in the latest (as far as I know) order that was finalized three weeks ago.
There are three specialty BP cartridge bullet moulds of which I am aware. The PRS is 45 cal, the Mav is 44 cal and the Snakebite is 38 cal.