I don't think you can.

You might ask the Fish & Wildlife agency if it's possible to obtain some and, if so, how.

As of last check several years ago in general dealers may not resell CA native herps with exceptions by permit for certain types of snakes particularly cultivated color morphs bla bla bla; meanwhile, if you go to Turner's or wherever fishing licenses are sold they have a copy of the regs laying out on the counter for free AND last time I checked your fishing license would allow you to take small numbers of such species as specified in those regs. Where will you take them from legally, in order to relocate them to your property, well that probably is a CA fish&game question However you may also wanna talk to rangers on BLM (Bureau of Land Management) land which is friendlier to USE than lotsa other public land. I'd like to do what you contemplate also (a toad-less garden really frosts my cookies, even if those are the least attractive american toad I can think of) so if you find an answer please post.