ah - well, if you've got the exact measurements, determining the volume is trivial. Your chocolate supplier isn't likely to have the densities for all their chocolates measured, and you can ask them to do it, which they will if they have time, or they'll make time if you're a large enough customer. If precision is important to you, remember that specifications have a range (ie fat is normally +/-1% on industrial chocolates) - so that range will result in a range of densities as well.

If you want to do the density yourself, you'll need a graduated cylinder and a scale - density is simply mass over volume - so fill your cylinder (on the scale) to 100ml of chocolate and read it's mass. D = m/v. The reading on the scale divided by 100 (the mls you used for the reading), and you've got a density calculation. Obviously if your scale is not calibrated, if you're not accurate in filling the cylinder, or if your chocolate temps are all over the place, it will impact your calculations.

Once you've got your volume of your cavity - simply multiply that by your density, and you've got the grams/unit of measure. Be sure not to mix your units of measure between your cavity volume calculations and density calculations (ie don't use both inches and centimeters, for example - one or the other)