According to my Buick parts book the rear wheel bearing roller assembly is a 115067, the outer race is 118189 and the inner race is 173684. My Chilton interchange lists the assembly as an 8078. This number interchanges to several currently available bearings. Any competent complete bearing and seal shop should be able to supply these. If they can't work from the numbers the dimensions measured accurately should allow them to make up the modern number as the numbers tell you the dimensions of the bearing so they are mearly working backwards.
I live in a town of 300,000 and our local industrial bearing an seal supplier has every ball and roller bearing for my car in stock so there must be some supplier near you that would have or could order in what you need.
You should get yourself a shop manual and a parts book for your car also an interchange book is great but they are quite scarce and very expensive. All three are often on ebay.
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Happy hobbying from Reid Pearce
"New Series Big Six" 6-30
Pontiac Custom Sedan "Tinindian"
Assembled on June 6, 1930 in
Regina, Saskatchewan, Canada