I always like to keep a spare [keyboard] on hand
It's a good idea having a spare keyboard. Years back, I learned this the hard way when one letter on my keyboard stopped working. And of course, that letter was one I needed regularly. It was my only keyboard, so I was basically without a computer (except for looking up stuff in document archives) until it worked again. Fortunately, a good cleaning fixed the problem.
These days, I keep spares of pretty much everything. I think it's a good idea, since my computer hardware is old--everything I'm using now, except maybe the mouse, is 10 years plus. If something breaks, I can get back in business in a matter of minutes. The only weakness right now is my monitor--my backup can only viably do 800X600. But that would be good enough to survive a few days.
For keyboards, one can go to a thrift shop. The thrift shops in my area often have them. Goodwill usually has a huge stack of keyboards. There is a good chance that it will actually work--at least something carefully chosen. I've even seen new keyboards and mice show up. The quality is pretty low--most of what I see are something provided for free by a computer company. It probably gets donated with little or no use in favor of something better. While not something I'd want to actually use for any length of time, a cheap Goodwill keyboard can be a nice, cheap backup that's "good enough" to get by with. And every now and then, a good keyboard does turn up--that's where I got the IBM M keyboard I'm using right now.