I'm not sure what your price range is like, but both of theses cage are pretty cheap but still an acceptable size for canaries :) a single bird will do great in it - plenty of room to fly back and forth.
https://www.amazon.com/dp/B001NPX5BC/ref=cm_sw_r_cp_apa_i_c9stFbGDNF05A
That's the cage I have :) it's great for my two birds. However, it's probably the smallest you would want to keep a bird in. They need more space than any pet store will let on.
Other than that, please make sure you purchase natural perches! Those plain dowel ones are horrible for their feet.
Some other essentials are cuttlebone, a couple toys, a swing, ane food! Some seeds are okay, but he or she should be offered fresh veggies and pellets as well.
Amazon is a great way to go, you can look at pet/bird stores but usually they don’t have the best ones for your little budgie, you’ll want a large and spacious one such as these that I found on Amazon Bird cage or one like This or if you want to keep them apart you could buy multiple ones like this Cage Hope it helped!Good luck to you!