For the best prince in the US, search the internet. (ie: Here on Ebay.com) For the best price in Mexico, search the internet. (ie. The small selection here on Mercadolibre.com.mx) In Cancun... there is an "HP" authorized store near OfficeMax, but they will more than likely just (see above) order it online and then double or triple the price for you. There is also a chain of camera stores called "Omega". There are ones in Plaza Caracol, Plaza La Isla, and one near the intersection of Yaxchilan and Uxmal. Their prices have never impressed me, but they might have what you need. Depending on the camera, it may be time to upgrade to a newer model rather than spending time and money on a replacement battery.
My last digital camera was a Canon and it had proprietary batteries. And even though I loved the camera it went through batteries. When I replaced that camera I bought a Sony camera that takes regular AA batteries. So now I can use rechargeable batteries. It's much much better, it's cheaper and the batteries last longer than they did in the Canon.
If you haven't found that battery yet let me know. I'm coming to Cancun from the States on the 26th. I can see if I can find one and bring it with me if you wish. Also I know a lot of people have mentioned problems finding things, like certain vacuum cleaner bags and other items. If you have something small, and legal, that you need from the States let me know and I'll try and pick it up for you this week before I leave.
I was going to add what Shari just said. Let me know, before the 27th. Shari we will be right behind you we are leaving MSP on the 27th. See ya there!
