XXX-Main sells a few different number decals (http://www3.towerhobbies.com/cgi-bin...2=MAIN&S3=&S4=
The decal sheet for the Losi Slider also has some decent numbers in various sizes (http://losi.com/Products/Features.aspx?ProdId=LOSB8206
Parma sells a number paint mask for 1/10 scale cars (http://parmapse.shptron.com/products...220.127.116.11?pp=15&
You can make your own white circle to put the numbers on your car with a circle cutter and adhesive vinyl - such as Monokote trim (http://www3.towerhobbies.com/cgi-bin...trim&search=Go
Since Monokote trim comes in multiple colors it can also be used as stripes on the outside of a car (hood/trunk stripes on a Camaro for example). When using Monokote trim as the rear stripe on a Camaro, it helps secure the rear spoiler and will help it from detaching/falling off the car during contact.
I've also used Carl Seils circle and number decals, which are very good. He also has decals to replicate specific VTA schemes such as the 42/48 'Cuda. He also sells a good headlight/taill light set for the Pegasus Mustang (The Pegasus bodies don't come with any decals) I believe the link in the post before mine has that info.
Also check with other VTA racers that are local to you. Many of the HPI cars come with extra decals - such as white circles, number decals, and even spare headlights. I've used plenty of the extra HPI headlight decals on other brand VTA bodies as well as various other stickers (door handles, contigency, grill decals, etc).