Where can I buy the 45N5864 RTU CMOS battery?

Just get a knockoff - unless the red and black are reversed, it's the same thing except without a premium, but you might need to replace it in 2 years vs 4-5 years.... But for 95% off, you take a chance. You do need to check them with a DMM to make sure it is wired right and isn't low (under 3V), but you can usually buy 2 for 10% of the price Lenovo wants. You can get it from Lenovo for... $78... for a CR2032 with a pigtail lead: https://lenovopartsstore.com/products/le... -- even if I was working on a T14 Gen3 or Gen4 surplus laptop, no thanks. Lenovo's smoking something with their pricing and margins.

eBay has plenty: https://www.ebay.com/sch/i.html?_nkw=Len... -- these are not even close to genuine but it's 95-98% cheaper. Some sellers don’t offer tracking at the bottom of the price spectrum but if you spend $1-2 more then that you’ll get tracking service as well. That’s something I would bother with.