FRU # to purchase replacement keyboard for Lenovo T450 20BUS05B08 laptop

You are in the US so you probably want the US Keyboard.  Check with your supplier about availability. If back order is too lengthy, eBay (as noted above) may be the best source.

I like to buy them new if I can.

Yes I need the US American English keyboard.

So then call your supplier (including Lenovo ) and / or use eBay

When replacing laptop keyboards (or display panels, for that matter), I much prefer to remove the old one first and look for part numbers on it. I've found in both cases that there may have been more than one option originally.

On the other hand, John appears to be knowledgeable about Lenovos, so it may well be that this is the only US keyboard offered for that model.

The US keyboard is out of stock at Lenovo.

Yes, but they may back order it, or as you noted above, try eBay.