I bought one 3 or 4 years ago that fits as well as the original telescoping mast antenna on my '69. I got it from an eBay seller in St. George, Utah. Problem is, I'm no where near my records and am not sure of who made it, but it may have been Counterparts. Below are a couple pictures. One shows the base of my original compared to the installed reproduction. The other is a zoomed in version of the only picture I can find with the antenna clearly visible. I'm fairly certain the original fixed mast AM/FM antennas used a different base if that's important to you. The antenna in this link appears to look the same as mine.
https://www.ebay.com/itm/1967-1972-C...RZ6h-c&vxp=mtr
Prior to buying mine, I had converted my original to a fixed mast. If interested, I can describe how I did it. Very simple and something to consider if you have an original in good shape.