Much like the crowds lining up to buy Snap’s new Spectacles, investors are rushing to grab stock in the owner of Snapchat before it goes public next year. But they’re finding little to buy as they scout the typical sources of stock—employees and other investors.

Those existing shareholders are rebuffing offers to sell, even though buyers are willing to pay as much as 25% above Snap’s most recent fundraising valuation of about $18 billion, brokers say. That’s likely because shareholders are aware that the price could soar after the IPO, when a wider group of investors will be able to buy Snap stock. Snap, one of the most anticipated IPOs in tech in recent years, is reportedly expected to be valued well above $20 billion in the offering.