Location, location, location. There is a computer recycler in Seattle, WA that saves Mac stuff to the side and Chuck nows how to configure them. Another option is to check for a Mac User Group in your area or thrift store.
The simple, obvious answer is to search online but you will pay a higher price for someone who has stored this stuff for years and still has inventory.
Post back with your location - name of city/country is close enough.
Your PowerBook 1400c has a single serial (mini DIN-8) port for either a printer or modem. With that in mind, I'd suggest that you find a used Epson Stylus Color 740 or 740i (blueberry/ice housing). I never ran an OS older than 8.6 with that printer, so check Epson's web site to be sure that the older driver supports OS 8.0. In spite of its age (1999-2000 vintage), it was the most versatile in terms of its (3) onboard connectivity options for a Mac or PC (Mac serial, PC parallel, and USB for Mac/PC). Additionally, the black and color ink tanks for that model are still widely available today, although expensive. I'd avoid buying a Canon-made Apple Color StyleWriter 1500, 2400, or 2500 printer, as well as an HP-made Apple Color StyleWriter 4100 or 4500 printer. Their hardware and printing capabilities can't begin to compete with the Epson 740.