Have Prov. Patent, Need Prototype/Marketing Plan

[Lucky]

I just sent in application for Provisional Patent on a Cell Phone accessory (used patent agent who did research, application, wrote up disclosure and cost about 1k)

So now I have a year until I need to continue with the more expensive utility application or lose rights to invention. Looks like I need to get a prototype and start trying to market it. I'm really hoping someone here has been through this process.

What is ballpark for a prototype (product will be much like accessories you see in store for 15ish dollars, fairly simple). Any good sites to search? My patent agent has recomended someone I'm also going to call.

More tricky is the marketing. Do I just head down to bentonville and knock on walmart door? Coldcall QVC and ask to try out?

My patent agent, like most i'm sure, is more into the product portion, and has few contacts/little info on marketing/sales side.

Any and all advice appreciated.