It is VERY common for pre-owned watches to be missing links that originally came with the watch. For two reasons:
1) The original buyer took out links and lost them.
2) The dealer takes a link or two out before selling. And then offers to sell you the links back at an extra charge.
😡😡
Although it sounds like #1 in this case since the dealer says they don't have extra links to offer you.
As noted above, extra links can be purchased from Omega. They cost about $50 per link. But it will probably take 2-4 weeks for them to come in. Don't pay inflated prices on ebay for extra links. Omega still sells them and then you know they are authentic.