OK, I spent a few hours this afternoon and got Mexico done as well.
You can find links to borders for both at http://www.soaringdata.info.

By the way, I spent some time comparing data from three different US
Government sources. There are some significant differences -- on the
order of 1000 feet or so -- especially along the river sections. I
guess it is tricky to try to pin down the position of the river with a
great deal of accuracy, as the meanders move over time. It is
probably prudent to give the US-Mexico border a pretty wide berth when
soaring in the area; some government official's map might not be
exactly the same as yours!

Lynn Alley
"2KA"