Major manufacturers include:
Kadee
Intermountain
Proto 2000
Atlas
JayBee
I've used Kadee, P2k, and Atlas before. All are decent. The flanges on the Atlas wheels sometimes are a bit too shallow for shall we say "realistic" trackwork. I've settled on Kadee. Good, solid, reliable, and USA made. No, they're not the cheapest, although theya re only marginally more spendy than P2k at the local hobbyshop.
If you're looking to replace a lot of cars wheelsets, look in the Walthers/Terminal Hobby Shop sale flyers for when they go on sale.