Thanks Dave, those do look like a great way to apply team numbers to bumpers.
The third link you posted may be the best bet for teams as the first ones may be a bit too tall for as bumper and the second one is pricey as it includes numbers teams will not use and a special coating that is only required if the fabric has already been printed on.
Here are two more sources that look like they might work (although you will end up with a bunch of extra numbers)
http://shop.hobbylobby.com/store/ite...0&ItemId=92682
http://www.shopsei.com/largeImage.cfm?product=881
You may also be able find something like this at your local arts & crafts store such as Michaels or Jo-Ann Fabrics