We take ours, we have a Garmin Nuvi which came with Europe and North America maps so we are lucky.

A lot of people have Tom Toms and take them. If you go here and select your model and then the select the South East region or USA if you want the whole of the US. You can choose to download it or buy the SD card. The information on what to do is here

I think some people have also bought from Ebay.

If you visit the US every year then I would buy your own maps, that way you can add your own favourites for next time :D