Emi
Usually through one of the portals, other than HGVC. They release the dates and you need to grab them nearly immediately as they don't last long. So you check all the time and you check sighting daily. You are also very flexible with your dates. You only get a studio at HGVC. I have never seen a 1 or 2 BR deposit from there. There are so few large units, that it does not surprise me.
So I usually grab them via my Wyndham portal, as most of the availability is outside of the 9 month window that the HGVC portal enforces.
Heat of Summer and Cold of Winter are primarily the only days that get deposited. The nice spring and fall days are all gobbled up by owners first and never hits the RCI Points pool.
Over the years I have traded into MC about 8 times, HC about 6 times. I have seen W57th but have not taken it because I like the HC location and breakfast.
I have occasionally used SFX for some of these trades, but they have recently put in a 1 in 3 year restriction on NYC properties.