I don't think the straw comes "chopped" since all the landscapers I've seen using straw for grass mulching are hauling a machine into which they feed the straw bales which are then chopped and sprayed out some distance from the chute.

I'd like to know where to get the bales of straw themselves to mulch my potatoes.

I was never able to find chopped straw or even just plain straw, but I did find a mention of a garden center in Glenside that has bales of salt marsh hay. I made the trip since it's not too far from my work and I picked up a few bales. So far I love the stuff.

Name of the garden center is Primex Garden Center. They have a lot of good stuff there, it's a shame I don't live closer to it, actually, I'd buy most my stuff from there if I could.

Here is a link that might be useful: Primex Garden Center

Try your local feed store. I live close to several and they all have straw, hay etc. If in doubt ask someone who owns horses where they buy their feed and you will find it. I actually prefer hay for my gardens but it is a little more expensive than straw.

I recommend you don't use straw. It has weed seeds in it and they will germinate. Use Salt Marsh Hay if you can find it.
It has no weed seeds.