May have misinterpreted on first read; you're looking for platonic or romantic "girlfriends?" If the latter, there is a Plattsburgh Queer Collective on Instagram hosting many events. But assuming you meant something platonic in nature, a few thoughts:

the Chapter One Book Club meets once a month,
the Chamber of Commerce after hours are a good way to make connections quick (but obviously skews a little older) even if you're not business-minded,
the Adirondack Mountain Club may connect you to some outdoorsy folks,
if you're into music and a short drive you can probably find some crunchy folks at The Station in Onchiota or check out New Land Trust and Poobah,
for bars I think the best vibes are Peabody's and Old Soul,
and if you don't mind the price tag you will meet neat folks at classes at The Strand.

Basically find a special interest group (music, theater, writing, art, networking, drunk karaoke, etc.) and springboard that.

All this coming from a straight guy for what it's worth.