There is a number of things you can use as weights. Things like solder, tire weights etc where ever you can squeeze it in.
To determine the approximate amount, I generally tape the model together in major sections as complete as possible. Balance it where the main wheels go and then add weight in the cockpit until it tilts forward. Add a little for good measure and build the model. As long as the weight is ahead of the main wheels, it should balance up ok.