Yeah, I've bought weed in Cali once, didn't have any problem at all finding CBD strains which are low THC (most would consider it trace amounts). Finding a strain with 0 THC (since I'm pretty sensitive and don't enjoy THC at all) is a bit harder.
The good news is, since 0 THC strains are legal everywhere in the US, you can just buy them online.
They don't show up in dispensaries much, because why go through all the hassle to meet regulations on a product which doesn't have to meet those regulations, unless they're being sold at a dispensary?
Yeah, that would definitely be difficult, as far as I saw you could get CBD powder at various places if you wanted to just add a little to a drink or do a dab.