Not to discount the others advise on bearing sources, but when you get to that point, call Accurate Bearing in Illinois. Do an internet search for the 800 number - (I'm not sure what the rules are for posting company info here). When you call, ask for Lynn, and tell her you are one of the "old tool guys". Read her the serial number off of your old bearing and she will cross reference her data to find you the modern replacement. I'm in the process of building my shop w/ all vintage tools, and I've used Accurate for all of my bearings - all of which have been a perfect fit. They stock all of the major brands, and their prices and shipping are unbeatable, (much much much cheaper than Delta).
Just my two cents worth.