Chevy dealers should have them in stock. Remove yours and take it with you. If you speedometer has been accurate, simply match it up with a new one. There are different colors which have a different amount of teeth.
If your speedometer is not accurate, then it's a matter of trying different gears until it reads correctly. For that, I would recommend a shop as they should have plenty of good used ones laying around for just this reason. Beats buying 4 new ones!
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63 Impala*66 Chevelle SS*69 Chevelle*69 C10 Stepside*71 Cheyenne 20*72 Chevelle
Parts for sale:
http://www.angelfire.com/trek/mr409/items4sale.html