What is the part number for a SIO connector?
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&amp;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.