I put into eBay: Dodge Plymouth 340 AC
idler pulley (note the spelling of idler)
I got this as the result:
Fits Dodge Plymouth AC Fan Idler Pulley CORRECT ALL THE WAY | eBay
Idlers can be replaced separately from the bracket they're attached to. Measure the diameter of the pulley, the width of the belt run in the pulley, and whether it has an integral spindle or has a bearing that goes on the spindle. Many companies make replacement idler pulleys for multiple applications, you should be able to go to the local parts store and match one up...
I put the same search into Google and got these results:
Plymouth Dodge 340 idler pulley - Google Search
I clicked on what looked like your pulley from your pictures - and even though yours is for a 340 and the pulley in the link says it's for a big block, I'd be surprised if it didn't interchange:
1971 Plymouth Duster Parts | MP600172 | 1969-72 Mopar with Big Block Engine & AC - Idler Pulley | Classic Industries
Here's one for "All Mopar V8 engines" as sold by HillTop Auto Parts
NOS Mopar Fan Idler Pulley 1955-73 Chrysler Plymouth Dodge 8 Cylinder Models - Hiltop Auto Parts