Hopefully this problem has been solved already, but yeah. I've been looking around for a couple of spare parts lately only to find that many bastions of SKS surplus now is dry and what's left is way more expensive than I'm willing to spend.
Dust covers are still out there but they seem to be mostly on second hand sites like eBay or Gunbroker or Armslist. As stated above, gunshows are probably OK places to look for them as well and if you're real lucky you may even find one someone will trade you for instead of outright purchase.
If you don't ever want to actually use the scope mount on the one you have, you can probably also modify that one to suit your need in a pinch but might be better to have a good second one in hand before you bust out the grinder.
Good luck!