I found an old bottle of muratic acid in the garage and used it as an etchant as already mentioned. It worked very well for exposed copper, but my paper stock had problems with the toner transfer part.
A quick warning: I'm not sure if there was some evaporation/concentration over the years, but when I transferred it to a leak proof bottle when done (the original plastic bottle was having issues), it was still outgassing a little. IT NEARLY EXPLODED THE PLASTIC BOTTLE. I'm very thankful I avoided true disaster with that.
Moral of the story: leave the lid mostly closed but allow for a little gas to escape the first day. A flip top plastic bottle (like some vinegar bottles I've seen) may also be a good choice.