I spent some time researching the same idea for our bumpers - a custom font in heat-transfer vinyl, and found the best price was at Stahl's (nice people)
http://www.stahls.com/heat-transfer-material for a custom cut design in their Gorilla Grip II. If you send them vectored art it minimizes the set-up fee. We found a local T-shirt shop willing to do the heat transfer for 8 sets of numbers for $20. A bit more all together than ready-made numbers, but not by much. Gorilla Grip II can be ordered by the sheet - about $12 worth will do - contact Stahl's to see what kind of cutters work.
For us, the issue came down to the idea that the team members are supposed to do the work, and iron-ons haven't fared too well. So we are going with painted on numbers. We've had trouble in the past with paint chipping and rubbing off, but after some research into painting canvas, we're hoping for good results from the following procedure: Cut stencils of our custom font. Use one coat of spray contact cement gone tacky on back of stencil to hold stencil to fabric. Seal the edges of the stencil numbers with ModPodge matte to minimize bleeding. Use a high density pigment acrylic paint (like artists' tube paint), 2-3 coats with a stiff-bristle artists brush, seal with 2 coats of ModPodge matte. The tests have been promising.