In addition to Suntronics Co. here is a list of other
manufacturers that I
know of who made add on keyboards, but some of these you also had to buy an
upgraded motherboard kit:
Kayde
Dk'Tronics
Haven Hardware
Memotech
Microcomputers Plus, Inc
The Full Spectrum
Synergistic Design
Fuller
Gladstone
E-Z Key
Research Applications Prod.
There were projects for proper keyboards for the ZX81, and I think the
Spectrum published in 'Electronics, The Maplin Magazine' in the UK.
Maplin sold kits for these (not suprisingly).
-tony