Looks like you can't get any now, unless you can contact Sprint or HTC, and if it's under warranty that might work. Otherwise you'd have to wait for broken ones to show up on eBay.
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[http://cgi.ebay.com/NEW-SPRINT-HTC-EVO-REPLACEMENT-LCD-NEW-OEM-BATTERY-/290444821260?cmd=ViewItem&pt=Cell_Phones&hash=item439fdc430c#ht_767wt_1139|Here's one you can get].
Looks like you can't get any now, unless you can contact Sprint or HTC, and if it's under warranty that might work. Otherwise you'd have to wait for broken ones to show up on eBay.