There are people holding out for an iMac with Retina/4K display. Unless you're among them, I don't see much point to waiting, and certainly not if your current iMac is seriously under-performing, likely to need an internal HD replacement in the near term, unable to run newer versions of OS X (older than a mid-2007 iMac model), etc. Having an updated computer for the upcoming year is going to pay bigger dividends for you than any improvements that are likely to arrive next year (a new computer that today is several times more capable than your current computer, vs. a computer next year that will be 10-15% faster than this year's).
My best guess for the upcoming year is that changes to the current models will be limited to storage configuration - more SSD/Flash Memory options and at somewhat better prices, and whatever incremental improvements to microprocessors Intel has in the pipeline (typically 10-15% annually).
When it comes to where to buy? It depends on your needs and where you are. The dedicated Apple department at your Best Buy may be your best, locally-available Mac-buying option. Or not. If you know exactly what you want, with no face-to-face sales support? Then you can treat it like a commodity purchase - shop around for the best deal. (If you're thinking that Best Buy/Geek Squad will also provide repair service... Geek Squad is not an Apple Authorized Service Provider - they don't do warranty service.) To search for Apple-authorized sales and support options in your area:
https://locate.apple.com
I don't see Apple bringing out an iMac with a 4K display quite yet - economies of scale in 4K display production have yet to be achieved, and the consumer market for $3K computers is pretty small (which is undoubtedly what it would cost at today's display prices). I just don't see Apple selling a whole lot of $3K+ iMacs to pros - pros who really
need 4K displays will also need more computing performance than iMac delivers. The first large-size Retina Display out of the Mother Ship will undoubtedly be a Thunderbolt/Cinema Display-style product aimed at the Mac Pro market. Besides, Apple's immediate goal is to get the pros to commit to the new Mac Pro configuration - they want to see fewer iMacs on professional desktops and more Mac Pros.
