Have you checked Amazon or the manufacturer of your sink?

Is there a reason you want a rubber/silicone mat? Have you considered a grid instead?

I did try Amazon and couldn't find anything as large as what I would like. I'm used to the smaller silicone mat I had from my previous double-bowl sink, so that's what I would like to continue to use. I may have to think about a grid if I can't find a large silicone mat. Just trying to keep the bottom of my new sink from getting scratched up. Thank you for your comment!

Look at silicone baking or tub non-slip mats and cut your own size and drain hole. They come in all sorts of larger sizes.

Saw a bunch of them at Home Goods.

I saw what I thought were fairly large ones at Wal-Mart, but didn't take notice of actual dimensions.