There are one or two specialist web sites which are listed on the links page on Phisick.com. eBay is a favourite haunt and there are one or two lists in most countries which are dedicated to medical and scientific items. If you have a piece to sell make sure you find these lists.
Pictured is an exceptionally rare "Fox's Leech Glass Cupping Set".
Note: Check Phisick.com for numerous images and complete details of each medical instrument shown.