Depends how strong a vibe she needs to cum. I can't cum with a pocket rocket and have to use a big Panasonic that is made for backs. We have a position I love where I'm on my back and he is on his side,like sissors, that allows me to put the vibe on my clit and him inside me.
Panasonic and Accu Vibe both make big rechargable models so no need to plug them in to use!
Also look online for what is available. Here is a good place to start:
http://www.goodvibes.com/