i have a 1st generation ipad, how do i order a charger for it?

Apple chargers are compatible with all generations of iPads provided you have the correct cable.

If you really have a 1st generation iPad(no cameras, domed aluminum back) it used a standard 10 watt USB charger. The updated version of that charger is this one: Apple 12W USB Power Adapter - Apple

See the following: Apple 12W USB Power Adapter - Apple

If you also need the cable, see: Apple 30-pin to USB Cable - Apple