Nyko makes a wireless sensor bar. Works pretty well, costs $20 at Circuit City or Best Buy.
Only real problem with it is that it takes 4 AA batteries (quite a bit, though you can get rechargeables), and that since it is separate from the Wii, you have to remember to turn it off when you are done with it. Nyko does have a timer built into it, so that if you forgot to turn it off, you can set the time to turn off the bar after 1 or 2 hours. So that's at least something.
I think there are extension cables (I found one on eBay) but they aren't official by any means. I can't imagine they are bad to use - it's just a power cord after all.