No, you do not need to buy the machine with an SSD drive in it, in order to install one later. Buy the SSD from another source and install it yourself, maybe putting the stock drive in a cheap USB case and use it for backup, etc.
Installing the drive is easy but you do need a No.6 Torx screwdriver, its a must! OWC has toolkits forsale online and your local heardware store should have them too. Keep the original drive and swap it back in if you ever have to send the machine back to Apple for repair, they like it in the original stock configuration, and have been known to swap put user parts, even lose them, replacing the part with a stock part! (Yes, I know, that's horrible, but true)
Another thing to consider is the lack of TRIM software on the Mac platform at the present, making longterm use of an SSD questionable. Search on the forums here for more info, but basically the SSD drives will slow down over a year or so, and it is not easily fixed. I would personally stay with a fast HHD for now, just my opinion.