I have a schwack of very soft lead..... Usually comes out a brinell hardness between 6&8....
I have found it very difficult to get cast boolits that show no deformities etc out of my current crop of 250(ish)gr moulds....
I find if I add Lino or mono to the lead mix I can quite easily get bullets from 14-18bn. These form quite nice boolits in my lee and Lyman moulds.
I'm not sure if this helps or not....
But I guess what I'm trying to say is, real soft boolits may be hard to acquire, mid range to hard cast are quite do-able.....
I have a partial box of Hornady 250gr "cowboy" boolits. I believe they came in boxes of 500.... They were a very soft boolit Iirc. Around a 10-12bn. Site sponsor "budget shooter supply" should be able to get them with out much wait.
I recently ordered up 500 255gr Keith style boolits from "the bullet barn". They specialize in "hard cast", I was told they would be bn22, but the ones I've tested were 17-18. They may be willing to cast soft stuff for you, but you would have to call.
I won't provide contact info as I don't believe that are a site sponsor, but the info is easily obtained via a Google search.
Good luck in your search!