What wifi extender will work for 2Wire707 380 1HGV?

The 2Wire 3801HGv is wireless a/b/g, so basically any wireless g extender will be compatible.

Some other options:

Do you have the ability to run a cat 5e cable to a distant location and attach a wireless access point or are you limited to a wireless repeater?

If you cannot do a cable run, another option is to get a newer wireless n router and disable the 2Wire wireless radio, using the n router, and you could even add a second wireless n router in repeater mode.