Here is an assortment of wheels available from Ace Hardware:
http://www.acehardware.com/family/index.jsp;jsessionid=mzthRQ8JryM0rfXGFMP7Tm7zFgPL5 DlZnVVmwt68tbHGSfmq82p2!11349649?view=full&categor yId=1260300&cp=2568443.2568444.2598675.2602604
To remove or install the wheels you just need two wrenches – one on the inside of the mower housing and one on the bolt in the middle of the wheel. Some wheels have small little plastic or metal hub caps that have to be removed with a screwdriver. Pull off the spark-plug wire before you put your hand/s under the mower deck – better safe than sorry.
Take an old wheel with you when you go shopping for the new replacement. I think that you can also find wheels at Home Depot and Lowe's. Of course lawnmower dealers could order you wheels – but that would probably cost you more.
With wobbly wheels there is also a chance that somebody has a bad bushing or ball bearing. In those cases it is possible that you could find new bearings – instead of replacing the wheel.
Homeowners also have to keep in mind the age of the mower, it's original cost and condition. It might not be worth putting on $40+ worth of wheels on an old mower in bad shape. Of course, in some cases, you only need to replace one or two wheels and not all four.