1948-53 216 and 235 main bearings are extremely scarce, but not impossible to find. However, 216 crankshafts are pretty available, since very few people have the knowledge or ability to work on a 216 engine successfully. Fitting the Babbit rods and aiming the spray nozzles in the oil pan have both become pretty much of a lost art. It's possible to do a little machining of the 216 block and run a pressure lubricated 235 crankshaft in a 216 block, but several other modifications will be needed to compensate for the difference in the height of the engine block between the 216 and the 235.
Replacing main bearings on a 216 is a very difficult procedure, since the crankshaft must be lowered almost an inch to get the upper bearing shells out of the engine. If they're in good condition, try adjusting the clearance instead of replacing them. Even if all the shims are gone, there's a way to tighten up the clearance. Drop me a PM.
EDIT:
10 minutes on Ebay turned up this ad:
www.ebay.com/itm/275642464377?Jerry
