When I searched for a MOSFET for use in my i4004 reconstruction, the only NMOS device I found that met my requirements was the Phillips (now NXP) BSS83. Production of the NXP BSS83 ceased in 2015.
In 2013 I found the SST211, SST213, or SST215 by CaLogic might be a suitable replacement, though the cost was significantly greater. Ouch.
When I search for the SST21x, I see four manufacturers: Siliconix, Vishay, Linear Systems, and CaLogic. Tonight I came across a press release from 2001 that explains some of my confusion over who makes these chips. It looks like the SST21x series was created by Siliconix. In 2001, Vishay, which by then owned over 80% of Siliconix, sold the the technology to manufacture the SST21x series to a company known as Linear Systems. So really the Siliconix, Vishay, and Linear Systems SST21x parts are the same. CaLogic claims their SST21x chips are a "proprietary design" so their FETs may be different but with nearly identical specs.
With the NXP BSS83 out of the picture, the SST211 or SST215 seems to be the best bet for a replacement. If you can afford to pay about $1/ea in 100+ quantity.