I saw some at Michael's, but they are hit and miss with inventory. Try Etsy? I bet you could find some in the "supplies" category? Or maybe an online store?
I use ribbon rings that Sharon sells at
Stamp Simply Ribbon store. They are perfect!! Here's a picture. And bonus -the girl who invented them lives right here in Oregon!!
Regarding the iron, I don't have a ribbon iron, but sometimes I run my ribbon across my lightbulb! Works like a charm!!