I've never heard of any sort of "bank" or anyone who has set up anything to generally distribute pollen. I think the demand is pretty small and pretty specific.

I think you want to communicate directly with someone who grows the species whose pollen you seek. For more obscure species this may or may not be easy information to find; I think one has to ask around.

I don't know how willing people are to send out pollen, but I would imagine the right person would do it for free (i.e. for postage). I don't know. Probably some people are willing to send pollen in exchange for a few of the resultant seeds.