The easiest and cheapest option for getting a new degu is adoption. People get rid of their degus for any number of reasons, from space and time issues to the behaviour being too much for them to handle to all sorts of other reasons. When you get an animal via adoption, you have a history from their previous owner and an idea of what your new pet likes and dislikes, but more importantly you have given the guy or gal a new lease of life and the chance of a loving home.
Places to look for degus to adopt:
In the above cases, you may be expected to pay some money. This will cover accessories in the case of Degu World for instance, or a small donation in the case of the RSPCA.
More sources, and possibly an updated list of degus to adopt, coming soon!