I'm the owner of the site you linked to (SIO2ST schematic http://mixinc.net/atari/pinouts/stxl.htm ). This connector is indeed a proprietary one, only used on the Atari 8-Bit line of computers (400/800/XL/XE).
However these connectors are still available from a few sources (like BEST Electronics or B&C Computervision) many people choose to mount such an interface into the Atari itself and just connect the few wires to the internal side of the SIO chassis. You can use a DB9 chassis and make a hole exactly above the SIO chassis to fit the DB9. Wire it in a way so you can use a standard modem cable to make the connection between the Atari XL and Atari ST.
You may have some luck via Best Electronics (apologies for the gaudiness of the linked site) since they seem to have tons of old Atari inventory.
Part number is CX-81 for the 5-foot cable. There is also a 3-foot and a 1-foot cable part number; see http://www.best-electronics-ca.com/ B&C Computervisions also stock there.
You can also obtain the SIO connector plugs from both sources (email for this; not published online).
As others have stated, these are proprietary plugs and are NOS (new old stock) or used. So far, nobody has generated free 3D printer models for these, sadly.
If you are semi-permanently connecting these, I would skimp on the cable and just use female to male jumper wires soldered to a pair of Ethernet cables (tho the SIO cables are cheap...).
Also see: http://whizzosoftware.com/sio2arduino/