Many people seem to make their own cages (sometimes using Ikea furniture). You can also find cages online.
I got my degus from the local animal shelter. There are many abandoned degus out there needing a home.
I'm in Canada so I don't know where to go in the UK (I assume that's where you are) but there are a couple of degu-specific internet sites (deguworld forum and degutopia) that have many UK members that can guide you regarding both buying/making cages and on which people list degus up for adoption.