If you start at the top of the page, and read through to the bottom you will become better educated on wheelsets:
http://cs.trains.com/TRCCS/forums/t/176675.aspx
Once you have done that and can decide on what code wheel you want to use (code 110, code 88, etc, which has ZERO to do with the code of rail you are on), you can try a manufacturer like Intermountain, Kadee, NWSL, etc.
You are running Mantua rolling stock. Did they make their own trucks, or buy from someone else like Athearn, or Atlas? Not all trucks accept the same wheelsets. If you do not know what trucks they used, you'll need to measure the axle width. The gauge of the wheels will be dead on across all manufacturers, but the width of the axle is what's important here.
Remove a handful of the current wheelsets from random Manuta cars that that you now own. Get a caliper (preferably digital) and measure the point to point distance from one side of the axle to the other. After 5 or 6 measurements, you should be able to get an average axle width.
Once you have that measurement, you can use that to determine whether Intermountain, Kadee, etc will make a wheelset that fits your trucks.
Here's a website that discusses many different rolling stock manufacturers and what axle widths are necessary for their particular truck.
http://www.theoldandwearycarshop.com/Reboxxcharts.html
If you need to find a caliper, try this:
http://www.harborfreight.com/4-inch-digital-caliper-47256.html
--Jon