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Many alignment shops that specialise in "competition alignments" will have scales that they use for cross-weighting/corner weighting. They would be able to give you a precise weight.
There might be scales at truck stops where you can weigh your car , cost a few bux though. I've seen a website where someone took a pic of their ITR on a truck scale and showed a weight of 2590 if I remember correctly.
if you want to do it for the hell of it, go to the dump or truck weigh station. if you're more serious about it, go to an alignment shop whose profession involves precision corner-weighting.