You can check with your printer for the specs, but usually the resolution required will be very low (often under 120DPI) compared to normal printing (magazines, packaging, etc). You can just purchase any image in XXL size and upsample in Photoshop, if required, to reach final size. You can look at most large prints in windows, displays, etc, they are rarely very sharp when viewed up close, they are meant to be viewed from a distance. Even a Hassleblad camera producing 50 Megapixel images would print at about 33"X20" @300DPI.
Also note that some sites that offer larger resolutions are in fact just upsampling the images for you. So you might as well pay for a XXL and upsample it yourself.
Hope this helps!