You can buy the knockdown texture in pre-mixed cans. 1 can doesn't go
very far and it's expensive (about $10 a can). I can't remember how
much it covers - pretty close to a 4 x 8 sheet of drywall, but it's a
cinch to use and matches perfectly. Spray it on and drag a taping
knife or wide putty knife across it to knock it down. I used it once
to match a new piece of drywall on my ceiling and it worked great. You
can also buy it in larger quantities that you mix with water and use a
sprayer on. Look in the paint aisle at Lowes or Home Depot and your
problem will be solved.

Al