Depends what kind of decals they are. Flames = Ghey. Any other ones are fine as long as they suit the flow/colours of the bike. When done well they look awesome and really make your ride your own. I know nothing of American shops but I guess Tapeworks.com would be a good place to start.
1) have a professional design the graphics. or copy one of the race bikes that have been done well
2) you can buy graphics from most sign shops , tape works, and eBay..
3) Price is going to depend on what kind of graphic kit you buy and what brand of Vinyl you want. Also the time it takes to reproduce what you want for your bike.
there are many kinds of vinyl from cheap crap Vinyl that will fall off in a few months to vinyl with 5-7 year life and UV inhibiters.
Vinyl Run Down:
Good Brands of vinyl media are 3M, Oracal, Arlon and Hexis. IMO
Crappy brands are Avery, Mactac, Vector and any house brand that is not in the list above as good brands.
Vinyl comes in two forms. Cast, Calendared and some high bread Calendared vinyl. Cast is the best and most expensive followed by high bread Calendared and Calendared. Otherwise known in the biz as High-performance, intermediate and promotional. Calendared vinyl shrinks because it is extruded in the manufacturing process and cast is not. The very best printed Media IMO is 3M IJ180CT with an over lam. And Orajet 3951 a close second.
The vinyl I used on my bike is Orajet 3551 and is a high bread Calendared print media, So far it is doing well.
The cost of graphics on a bike will run from a cheap ass job of $200-$300
and Up to as much as $2000 for the real deal.