Etsy shop. They are large, 12 mm stitch markers that can fit up to a US 17/12 mm needle. The picture below shows them on a US 11/ 8 mm knitting needle, so you get an idea of how well they work for chunky and bulky knits.
They also work equally well on smaller needles- I used a similar pair successfully on US 8/ 5 mm needles with sport weight yarn and knitting went beautifully.
Oh, and the sale- enter "ART01" into your Etsy checkout cart for 10% off until Valentine's Day this Saturday. :)