Here's a UK-available
universal DVD battery, probably of the same manufacture as the options many people are using for the R-44 and other portable devices. I'm certain you can find a better price if you look around.
As I don't have an R-44, I'm unsure of the proper tip connector. But it should be pretty easy to find, once you determine the proper size. Hopefully someone else will chime in on the tip size and polarity.
Worst case, you could always use the wall-wart tip (which I've done in the past, on occasion, if I know I won't need the wall-wart). Easy to hack, using wire nuts (no soldering required), and easy to swap back to the wall-wart if you need it. Not the most elegant solution, but durable, field-serviceable, and simple.