I'd like to pick up an extra battery for my Nikon D40. The OEM one is this for $37, but I can get generics on eBay for around $12 or even ones that appear to be OEM for $16. Is it safe to get non-OEM batteries? I ask because some electronics seem to be very particular about that sort of thing, and some aren't. Does anyone have any alternative suggestions for good deals on batteries?