I looked into this extensively last year. Unless you're willing to order a coupled hundred dollars worth of decals, your options are limited.
SSSink probably does the best job, but... they don't answer emails promptly (use the phone instead), and they won't sell you just one or two decals. Everybody else is, basically, selling stickers: they're very thick and the adhesive seems pretty cruddy (based on the samples I got). If you go this route, I'd order a bunch of stickers since most don't look like they'd last more than a week.
At this point, I'm wondering if the best option for a one-off bike isn't to just hire a painter. Seems like it should be pretty easy to shoot some lettering through a stencil and then spray a clear coat over it...
P.S. If you find someone that does good work at a reasonable price, please let us know!