really you have replacement necks, generic necks, custom made necks and special material necks.
So there is a lot to chose from, depending on what you need,
So if you need another type of neck or a replacement to your Yanagisawa, Yamaha, Selmer , Keilwerth neck you go to the same sources for saxophone.
On ebay, you can find inexpensive generic necks. Some brands make their own version of a generic neck (Barone, Oleg, and many others), so they are not " custom made " really and generally are made to fit either a Yamaha or a Selmer-like saxophone you (a good tech that is) can shrink or expand the tenon to fit your horn.
Then you have a real custom made neck. Bosken and Gloger are some examples of this. Gloger makes necks for several brands and types. In the beginning his necks were replacements then they became necks with their own identity and you can ask a different design to achieve, if at all possible, different results.
There are special material necks too like the Paraschos, made of wood and fitting different brands.