Pudge

I just finished sewing 3 covers made out of a packing blanket on my 15-91. I used a size 18 needle which was the largest size I have on hand. But I note that my manual says to use a size 21 to sew "very heavy materials comparable to sailcloth, denim, ticking".

I could have used a larger needle to complete the job. Does someone know if size 21 needles are sold and where I can possibly buy them?

Cheryl