I had a customer request a pillow insert that is 24" x 16" and it's a bit of an oddball size so I don't have one in stock. I looked into it and they're at least 17.00 on average and I thought wait a minute....
I went to Walmart and picked up a regular old pillow in "medium support" style for 4.88. This is the standard size and it's 17 x 26 inches.
I cut the end open.
Removed the polyfil stuffing.
And then cut it to the size I needed plus the seam allowance (16 1/2 x 24 1/4 since it already had a seam at one end).
I stitched up each side and a little bit on each side of the end. I left a decent sized hole to stuff all the polyfil back in.
I grabbed my cheapo scissors and trimmed off some of the polyfil at the end (I'll keep the excess for a pin cushion or softie project).
Then I stuffed it all back in making sure I got it into the corners.
I pinned the end shut but put the pins about 2 inches from the edge so the fluff wouldn't get in the way while I was sewing. All of my seams are obvious but since it's going inside a pillow cover it doesn't matter.
Bam! It's exactly the size I needed and cost less than buying it even at a wholesale price. You can do this with any size of insert you need.
Give it a shot and let me know how it works for you!