Buying whole set of wheels is definitely more expensive with prices like approx. 12 - 20 $ per pair. A pair of donuts is about 4 – 10 $ that is half the cost. You can true the donuts to your desired size or start at high rollout 55mm (plenty of life on donuts) for bashing on tarmac.
I 've got plenty of rims too, always cleaned and ready to be mounted. LRP, HPI, Kawada, GQ and some other asian brands are reasonably cheap. I get them from hobby shops in Malaysia, Singapore & Japan.
I use rubber glue, the type cobblers use, caveat emptor, some kids use it for sniffing. Glue them evenly before you true them and they won't be off balance or come apart. Factory mounted wheels go through the same process but only different level of workmanship.