The choice of models "easily" available to Americans by mail is very narrow compared to what you can obtain by visiting Canada or having a Canadian friend/relative buy it and ship it to you. The 550, 650, 560 and 660 are impossible to obtain without going to Canada yourself or asking a Canadian to buy it for you- period. These models are in high demand, when they get returned by clueless buyers they are immediately marked down as "open box" and blown out of the stores. They are not sitting around in quantity so some internet liquidator can come along and get them for you.
On eBay or on various electronics import sites, you will occasionally see a 550, 560, 650 or 660 but the prices will be outrageous, close to $1000, and these are generally not the Canadian/North America version but the complex "international" models with oddball tuners and connections. There are members here who heartily recommend such "international" units, but I think they're a rippoff unless you are utterly desperate and have money to burn. (The Pioneer international units have particularly strange timers that require tricky settings to even make them visible on a USA television- beware.)
The North American Pioneers most frequently seen on eBay are the 540 and 450 (eventually we may see the new 460 pop up as well). The 540 is a 640 with a smaller hard drive (80GB vs 160GB) and no USB socket. The 450 is identical to the 550 but does not have USB or DV sockets (it does have HDMI). The new 460 has not been liquidated yet, it is identical in every way to a 560.These appear often on eBay because they are special models Pioneer makes for warehouse clubs who buy thousands of them at a time to put on sale. Of course the typical warehouse customer finds a DVD/HDD recorder "too complicated" and returns it almost immediately, so CostCo etc winds up sitting on hundreds of brand new "opened box" Pioneer recorders it can't get rid of. Eventually an electronics liquidator will buy dozens at a time from these warehouses, and put them up for auction on eBay, where on average they sell for $250-325 including shipping to USA. I have bought several of these over the years and never had a problem. If you can get over the psychological hurdle that someone else opened the box before you did, they are an excellent buy and the easiest way for an American to obtain a current Pioneer recorder.
There are two liquidators on eBay I have dealt with successfully, one is in Montreal and is a little hard to communicate with because their French is better than their English. The other is elsewhere in Ontario and is very easy to deal with. I'm sure its against forum rules for me to recommend an eBay dealer by name, so I'll just say he's easy to find by searching for "pioneer dvd recorder", looking at any of the DVR-450 listings that come up, and zeroing in on the seller whose "handle" is his three letter name combined with a four digit number. He generally has one or two DVR-450s listed every ten days, although I don't see one today. His listings never change- there is a picture of the unit and its box, and a narrow column of text describing what "open box" means and detailing a one-year warranty.