Dutchman, really any of the 9v li-ion DVD batteries(typically 5400mah and up) should work just fine. Initial, Dynex, Impact, Endurance are a few brand names that are known to work with the R-44, don't know if any are still in production, but can often be found on Ebay.
Otherwise like ToddR mentioned, make sure you have the R-44 set to "adapter"(as if using a/c power) and not one of the battery settings. The low battery threshold is just too low for any of the external battery settings to work with our 9v options.
If you still have problems, I have 2 or 3 extra 9v 6000mah batteries + charger that I used with my R-44 that I will let go cheap, shoot me a PM if interested...