Your best options are probably the EZ Reader, which supports DRMed MOBI ebooks, or the Sony PRS-505.
The EZ Reader is new, but it is just a re-branded Hanlin V3 (like the BeBook and others) so we don't expect any surprises. If you want to be sure, wait until some of us get one and report back. The post on how to buy today is
here.
The Sony PRS-505 is a good choice because it is available in thousands of stores in the US, so you can probably get someone to pick it up for you. There is are issues with buying ebooks from the Sony store without a US credit card. These can be worked around (Sony Connect gift cards, sold at Borders and Target stores in the US), but I suggest buying MOBI or LIT (MS Reader) ebooks instead and stripping their DRM. DRM-free ebooks of any kind can be converted for the PRS-505 using Calibre (which has its own sub-forum). I don't know what the legal status of DRM stripping and format shifting is in Mexico, but doing this for personal use on ebook you bought harms no one.