J battery won't fit the E90. The J battery is a 9.6V made with 2/3A mini cells and designed for the TM P90. Echo 1 have made a small adjustment to the internals of the battery compartment allowing the use of a side-by-side 9.6V mini battery, like the one that comes with it.
However, those aren't typically available. I would recommend going to www.cheapbatterypacks.com
and build yourself a custom 9.6v 1500 mAh pack using Elite cells. It will cost about the same as a cheapo battery back would, but it's made with the best cells on the market and quality wiring and workmanship.