Used cages are great, just make sure to clean them thoroughly before adding rats :). I have this cage from MCage (https://www.amazon.com/gp/product/B0B5CJYJ11/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1). It holds 3-4 rats with some room to spare and I've been very happy with it.