When I converted my double Grey Shimano Exage crank to a single crank and the local shops didn't have single crank chainring bolts in stock I just cut down the female portion of the existing crank bolts. They work fine. Here's an image of the converted and polished crank on the bike.
Oh, if you get single crank chain ring bolts, they might be for a 1/8" thick ring and a derailleur chain ring is 3/32" so you might need a thin spacer or file down the bolts a bit.
Cheers