Never did a Ryobi, but I have rebuilt a few DeWalt and Milwaukee packs over the years.
Digikey carries a good line of NiCd and NiMH cells (including the high capacity ones for power tool applications), with solder tabs prewelded onto the ends. If you give them a sketch of how you want them assembled, they will even solder and shrinkwrap them into custom assemblies for you.
http://www.digikey.com