as for cleaning- hot water and any good oxygenated cleaner (oxyclean, onestep, etc) will be fine. take apart the dip tubes and flush them out to remove syrup/etc. rinse the o-rings well or buy new one(cheap). rinse the lid and its rubbers. use hot water and a brush or a scouring pad- but not metal stuff like copper/stainless/brillo scrub pads.
i've bought soda kegs from both bev elements and adventures in homebrewing/AIH. where you order depends on what you're looking for in terms of kegs. you can buy new kegs, but that's pretty expensive. like $100 or so for a 5 gallon. but for used? out here in san francisco area the going rate for used 5 gal keg is $50-60 bucks on craigslist. i'd assume its similar in your neck of the woods.
so-- new or used? these kegs are durable goods that last a long time with moderate maintenance so for me its used kegs. as far as who to buy from, in my opinion if you want standard 5 gallon kegs then just go with whoever has the best price after shipping (if you cant find any locally on craigslist or homebrew forums). then your only real decision is to buy pin lock or ball lock style keg.
at AIH they sometimes sell a package deal of 3-4 used kegs at good pricing, maybe find a buddy to split with purchase with?
also- you can also get 2.5 or 3 gallon kegs if you brew smaller batches or for variety's sake. great for splitting batches up for different dry hopping, adding spices/fruit, making sour/brett, etc. AIH is better on pricing for these sizes in my opinion.
but if you absolutely need one RIGHT NOW then do a good craigslist search and check "for sale" stuff on all the internet homebrew forums first. so if you cant find one that's close to you dont sweat it, they're all pretty much the same thing. just go with whatever makes sense for price/speed.