A great rule of thumb is 2 square feet per rattie. Some breeders keep theirs in only 1 square foot (like labs) but I like to make sure our ratties are get 2 square feet.
A couple great places to find used cages is on Facebook or Craigslist. We found our favorite flight cage on there and relisted it when we finally were able to upgrade to Midwest Critter Nations. Please keep in mind, something that many overlook is the bar spacing. It is very important that the spacing not be more than 1/2". A few have been able to keep adult boys in 1" bar spacing but most often younger males and most all females will be able to escape. If you find a cage you LOVE but has the larger spacing you can remedy this by adding hardware cloth with a smaller spacing. This isn't ideal (hard to clean and lots of sharp edges) but it is doable.
Below is a sampling of some popular cages with the maximum number of rats that should be housed together in each cage. Please keep in mind, some rats want/need more space than others. Though females are smaller they are active and need the space to explore. Males can get big, especially ours! So make sure you take this into account when you are looking to get your cage.
Bedding is something that is as varied as the types of cages available. In our TCNs, we use fleece with puppy pads or cotton pads under to help wick away the moisture from urination. Several others use flannel in their critter nations. All of this works well when the rats are also litter trained. Here is a link to litter training. Another option is use a bedding that is available in stores. Some like paper bedding, some like wood shavings and some use pellets. Most important is to not use cedar. There are many wood options and if you do chose to use wood shavings or pellets, please do not use any unless it is kiln dried.