If her cage is large enough for a litter box, then you should start using one. Since you said it was a small cage, you may not be able to fit a litter box in there, so you could try some of the following. It really would be best to get her a larger cage or an x-pen and start training her to use the litter box.
If you have any small fleece blankets, you could put one of those in the bottom of her cage, then wash it (each day?). You would need to have more than one, so she would have something to use while the other one is being washed. You could also use a towel, but those are more dangerous if they chew on them and ingest the fibers.
You could use shredded newspaper. It would be free, if you or someone you know gets the paper. It’s not ideal, but if it’s just for a few days, it should be okay. Or maybe you could buy a small bag of bedding, either the brand you normally buy, or something that looks similar. I have seen small bags of paper bedding, similar to Carefresh, for a low price at the grocery store. Try some discount stores and see if any may have a small bag for just a few dollars.