Had the same problem with the spare key that came with my RX300.
Bought this http://www.ebay.co.uk/itm/Lexus-IS200-GS300-LS400-RX300-3-Button-Remote-Key-Shell-Case-Short-Blade/401138099983?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2648. had the blade cut at a local shop, fitted a new battery to the old fob and swapped it over, worked perfectly,
£15 total,and i have a spare battery
The guy in the shop did say to be careful of the cheap ones from China, as the blades tend to be too thin or even hollow, making it impossible to cut, ;-)