I have no idea about the proximity of any Walmarts but you can always go to Google Maps to see which one is closest. Also try Target and K-Mart if there isn't a Walmat nearby.

I'm not sure I would bother with the taxi, particularly if the driver has driven in the US before. Why not just rent the car, have one person drive to Sprawl...I mean Wal...Mart to buy said car seat (with everyone else waiting at the rental place) and then return to Alamo? It would save you the taxi fare. You might also want to bring/buy/request a GPS system to speed up going to/from Walmart.

Hi thanks for the info i think that is what we will do,find the nearest store and get back to the
car rental, seem,s to be the best way
thanks annx

Google maps show a Superwalmart at NW 13th terrace, which is a 5 minute drive from the airport.
Wait and see what condition the Rental seat is- if it is safe but looks very second hand, just cover it with a crib sheet.