Sadly, in New York, there are just a few remaining shops that have a wide inventory of gear like thongs etc. Not like years ago when there was a treasure trove.
Best best is to walk along Christopher Street in Greenwich Village and Eighth Avenue from 14-23rd streets and step in the remaining shops.
https://chelseaexclusive.com/
http://rainbowstation.nyc/
https://www.instagram.com/theleathermannyc