The Sideways RAM unit, developed by Paul Townsend, comprises 128K of static RAM and an interface to the Amstrad computers, allowing software destined for sideways ROM to be developed easily.
It wasn't so much "bought" as a workable unit, but was designed and built by Paul himself, inspired by the scrapping of the original RAM board. The interface logic between the CPC and the original RAM chips, was all his own design.
Essentially it was a "soft" version of an 8-chip ROM board, a development aid for ROM-based software. At the time Paul was developing a ROM-based word processor for the CPC. Not having PROM burning facilities at home, this unit was ideal because new versions could be assembled, tested, debugged and developed further while reproducing the target architecture.
In the end, it proved more reliable than a true ROM board since there were no hardware contacts to go haywire (though there were software bugs). Paul ended up using that exclusively, loading up a standard configuration into the sideways RAM with different ROMs, along with Maxam and Utopia, with Oddjob, BCPL, Toolkit and a few others to hand if required.
Sideways RAM schematics: