Morning Brian,
You would do best making your own. Go to a good quality artist's materials shop and buy a couple of sq metres of fine, matt black cloth about 2/3 mm thick. You may find similar at home. (Blackout material).
Then simply cut the cloth to size and use double sided selotape to fix it to the desk. The Artist's cloth costs about £1 a sq metre. I made a similar mat for that price and it is still good 3 years later.
Regards, 71.