My drawer. There are a bunch in there.
More seriously hunt around yard sales, thrift stores, flea markets and such places. Depending upon where you are (more populated is better is but you face competition) then you will often find such things around there (you may have to ask), though right now it is late in the year and not so many people are doing such things. Ebay, amazon and the like people seem to have bumped the price for reasons I am never sure about (retro seems to be cool again) and seem to want around $25 USD for anything common and more if it is rarer colour, I did not even look to see what the lit screen model that appeared in Japan goes for. Likewise I am not sure what goes as far aftermarket screen backlights/frontlights for the SP.
Alternative. The GBA SP is basically the same form factor, plays the games (including link games), has a light in the screen (original GBA does not unless you add one) and is more readily available. Bonus if you don't care for link games is GBA flash carts can emulate the GB/GBC pretty well and GBA flash carts are easy enough to get and use.
You might find some GB/GBC clones out there in the world. I am not sure I can suggest any of them at this point, a pity really as that would be nice.