I used one similar to this and had no problem. There should be many suppliers and possibly better prices.

[link=http://www.all-battery.com/72v3800mahflatnimhhighpower38adrainratebatterypack swithtamiyaconnectorsforrccarsandsumorobots.aspx]all-battery[/link]

You can check the dimensions given to be sure:

"Standard Dimension: 5.39"x1.87"x0.95" inch, fits all RC10 cars"