I read a lot of the old PC Mag on google books, so this is what I Can deduct about that time period.....
The biggest companies leased their gear from a big manufacturer (ie. Compaq, HP, DEC, or IBM). The biggest name (And probably easiest to come across) are IBM's PS/2 series, followed by Compaq (Deskpro 286/386/486), then HP, then DEC. Back then, it was a common held belief that "noboby got fired for buying an IBM". Other (but lesser used) brands at the time included Zeos, Acer, Northgate, Gateway 2000, and Dell.
Local companies tended to stick with locally built or in-house built "White Box" computers....meaning they were built from parts that came in generic white boxes in bulk, and were often built to fit a certain job role...ie, an engineer using PC based software in 1992 would likely have a 486 DX2-66 running Windows (for workgroups) 3.1 or DOS with some Novell network on it, 16MB of RAM, and AutoCAD, whereas the secretary probably got a 286 running a menu (or DOS Shell) with access to WP51, Lotus 1-2-3, and DBASE III.
Also, a secretary or someone doing basic office tasks would probably have a monochrome monitor of the office is budget consious, whereas the engineer could have a bigger monitor.
In aesthetics, Almost everything was some shade of beige in those days, and at the most elaborate, would have darker shades of beige for the other parts. Black floppy drives were starting to dissappear as were externally visible hard disks with black face plates. The cases were boxy, had at least 3 LED lights on them, keyswitches, and buttons, and the monitors were on average 13-15" big.
Also, some companies DID have an elaborate mix of machines. Back when I worked at a major aerospace manufacturer, I knew a guy who had been there since the 80's, he told me back in the 80's-90's, most people bought their own computers and brought them to work....having a mix of machines, so uniformity may not have been the norm for all companies at the time. They might have the grpahics guys working on Macs, the Secretaries on Typewriters, and the Admin on 286/386/486 PC's. Also, computers were expensive so even some early 80's computers could still have been around at the time.
As for finding the machines, might want to look into some of those recycler centers, thrift stores, or maybe some corporate/government auctions of bulk PC's. That's wher I'd look if I were making a period piece and needed a pile of 486's to poulate a retro office.