I assume you mean a session where everybody mainly sits in and jams together, as opposed to an open mic/open stage situation where performers take it in turns to get get up and do their bit in rotation - like a folk "club".

I suppose you could just take a punt and get a room in a pub, put up some local adverts and see what happens. However, it might be better to (a) get some ideal of who, locally, might be interested, and (b) see if there's a possible venue. So - why not have a word with the landlords of the pubs in your area and see if they might be up for an acoustic session in the bar. If they know there's no money involved, they might well say "yes" in principle. Then advertise locally to see if any musicians are interested - and get to some local clubs to see if anyone's interested.

If you do actually get a venue and some musicians lined up, then I think you have to advertise it as an open session - which means musicians of all shapes, sizes and abilities. That's what it's about.

Best of luck!