If it were me, I would post a "want to buy" ad on the AK and Saiga forums and see if I get a response. Before KVAR sold out of the Saiga 7.62x39 bolts, they were $125, so if someone responds to your ad and they know the market, I would expect a price of $150 or more, especially now that Saigas are banned from importation. A related option would be contact builders that convert saigas (Arsenal, Krebs, Rifle Dynamics, etc.) and ask them if they have a Saiga bolt that they're willing to sell you.
Opening the bolt face on a 5.45x39 bolt might make sense, assuming that they have the same lug dimensions, especially if you have a suitable 5.45x39 bolt. If you go this route, the extractor in the 5.45x39 bolt will have a different radius than a 7.62x39 cartridge. However, I have fired a couple of primed 7.62x39 cases with a 5.45x39 extractor in a Saiga 7.62x39 bolt and the extractor worked. Don't know what would happen after lots of rounds. As of a half hour ago, KVAR still had Saiga 7.62x39 extractors in stock for $25.
Another option would be to buy an unconverted 7.62x39 Saiga. Current market price seems to be about $600, but since you're getting the $150 bolt that you need, you're essentially paying $450 for some spare parts which is better than having your $1200 investment sitting in the corner.
Good luck,