This is what I ended up with.
I never saw the Sprint 2600 mah ones anywhere. I found it on EBAY. It came with the HTC 2150 mah battery and a nice fitting cover. It looks exactly like the Original one on my EVO but it doesn't say HTC on it so not sure if it's a true OEM as it says. Going by the BatteryBoss.org site, which everyone seems to, the HTC 2150 is close to being a true 2150 compared to some of the claimed 3500's out there.
Also to me the 3500's are just too big. I never really had a problem with the 1500 but I liked the security of the 2150 for the days I'm on a jobsite and definitly can't charge it during the day.
That is not the buyer I used but it looked similar. I bought from cell-force on Ebay. they don't seem to have anymore of what I got on Ebay at the moment. I'm sure you could contact them and ask questions about their items.
Hope this helps,
Nate