There's info on DIY wooden rat cages online if you're still going ahead with that. I'm not sure if wood sealer is safe to use. What kind of wood are you using? Some wood are harmful to rats.
Critter Nations are really popular if you end up getting one
I have a double... for one rat 😅 There are other similar cages such as Full Cheeks (a Petsmart only brand) and Exotic Nutrition (pricey) and Prevue. Amazon Basics apparently had a small animal cage that is nearly identical to a double Critter Nation at one point but it's been discontinued. I saw a mention of accessories for it on an Etsy shop. Look around on FB Marketplace, FeeCycle, Buy Nothing, etc for used cages.
Are these your first rats?
Critter Nations are really popular if you end up getting one
Are these your first rats?