Siafunds aren't available on normal exchanges. The SEC has designated Siafunds as securities according to United States laws. We recommend that you research the applicable securities laws in your country before buying or selling Siafunds.
If you are based in the United States, you may be required to be an accredited investor to buy or sell Siafunds. We recommend consulting with an attorney.
We treat them like securities, and most exchanges aren't willing to follow the regulatory requirements necessary to trade in securities.
There are however Siafunds markets on Bisq, a decentralized exchange. You can also check the Sia Discord server for the #siafunds channel, where over-the-counter trades are available through a trusted escrow.