I don't think you will be able to buy 20mm t-moulding as 20mm isn't a standard board thickness.
However, you can get 3/4" (19.05mm) and 13/16" (20.368mm) t-moulding.
I'm not sure how accurate you need to be. Your sheet thickness and the t-moulding are not likely to be bang on anyway due to manufacturing tolerances.
I would normally suggest t-moulding.com but I was just having a google and found [
this UK t-moulding supplier]. Maybe you could give them a go?
They say they stock 3/4" and 13/16" t-moulding but you have to check with them which colours they have in those sizes. I'm guessing they will have 3/4" in leather texture but not 13/16".
Just one other thing, note the different spine widths of the t-mouldings.