Something about 25th Street screams "collectibles." A stone's throw away from one parking lot flea market is an antique's garage, a cluttered paradise of used goods.
As a fancier alternative to the outdoor market or busy indoor one, try the Showplace Antiques Center, located right across the street from the West 25th Street Market. A high-end affair, this antique and design center features four floors of jewelry and furniture, ranging from showpieces you wouldn't let a dirty child 10 feet near to every day, usable kitchen furniture.
Over 150 vendors use the Showplace to sell their authentic wears, which on a given week predominantly lean towards jewelry and decorative items (think lamps and pottery).
Get a sense of what the Flatiron space has to offer by perusing its online catalogue. The offerings will be sure to wow you with both their uniqueness and randomness (disco ball, anyone?).
The best time to shop in person are on the weekend, when you'd find a majority of the dealers setting up shop. As with most markets, the early bird gets the worm, so don't wait too long before shifting through the wares. Of course, if you don't find anything you fancy on the Showplace's floors, you can always head down the street to browse 25th Street's flea market alternatives, or leave that disco ball dream for another day.