Critter Nation cages
are really popular. You can start with a single and add on later or go with the larger double (basically two singles) and add on from there. If you look online, you'll find that some people have like an entire wall of 8 cages (4 doubles connected to each other). The same company makes Ferret Nation cages which look similar but with vertical bars that are more widely spaced apart. Good for ferrets but not for rats.
has similar cages but they're more expensive.
Petsmart's Full Cheeks line
has some good cages.
Prevue has a small animal cage
that might be ok. It just has vertical bars which doesn't allow for climbing but you can just put in wooden ledges and rope and other things.
If a brand new cage is too expensive, start saving money or you can look on FB Marketplace, FreeCycle, Buy Nothing, etc for used cages.
Oxbow seems to be the most popular brand of commercial rat food in the US. The Essentials line is recommended. Food Archives - Oxbow Animal Health
Places to buy rat cage accessories: Suggestions on stores for supplies? (US)
DIY toys: cardboard tubes, cardboard boxes, lots of other ideas on Pintrest and YouTube
General rat info if you're completely new to owning rats:
A non-profit international Rat & Mouse club for show and pets