I just googled Long Island Aquarium Society
link here
Home
I am assuming there is only one LI...
Otherwise there were other aquarium societies in central, Brooklyn and others.
When I joined our "local" one, it was recommended to me by other hobbyists. I read about the location and meeting times, and it said visitors were welcome. It took a year for me to join simply because the meetings are far away and usually on a day/time I work.
Ours also has mini auctions each meeting (one time per month) and the members bring fish, inverts and plants, to sell off to the highest bidder. Always nice quality fish and shrimp, other stuff and good deals.
Ours has the best auctions and it was about $25 to join for a year. That helps pay for meeting locations and featured speakers and the costs of setting up the auctions and keeping the website going, etc.
I am guessing it would be the same. You can always use the contact feature listed. Looks like they have a social media page too. Otherwise there are often local FB group members who may be able to point you to the right direction for local fish.