Good source for genuine batteries
I always buy batteries on eBay.  "But that's not reputable at all," you might say.  Indeed.  However, I find that as long as you search for the specific part number (e.g. BB99100 for the Nexus One / Desire battery) and ensure the posting contains pictures and guarantees it's genuine, then you'll be OK.  EBay sellers avoid lying about that sort of thing because their reputation will quickly be damaged.  Also ensure the seller has a good feedback rating (97%+).

Here's the cheapest new battery from a very reliable-looking seller that I could find.