Truck stops, Moving companies, Grain Elevators typically have scales
capable of weighing your rig.  Some state weigh stations leave their
scales on when they are closed and you can go in there and weigh.
Don't pull into a state weigh station and ask for a weight though -
they are in the "money-making" business with the big trucks, and you
could lose some money if you don't know that your rig is overweight
and they find the problem for you.

When you find a place, make sure you get the axles weighed
individually - Tow vehicle front axle, tow vehicle rear axle and the
two trailer axles together.  If you want to get real picky, try
getting separate weights for the individual trailer axles, but that
can be a tough split sometimes.

Some scales have split platforms, so all you have to do is drive on
and ensure each axle is on a separate platform.  Others are a single
platform, which means that you'll have to drive on one axle at a time
and have the operator make separate weights for each and then do the
math to figure out the numbers.

Typically, costs were running about $10 last time I checked.

Greg

In addition to the other types of businesses mentioned in this thread, you
could look for places that buy scrap steel and similar material.

Don't forget rock quarries.  Sweet talk the lady in the scale house when
they're not busy, and you can probably get an axle by axle reading for free.