You don't know where to issue because you do not know where he is? If so, then you can send it to his last known mailing address and maybe he has his mail forwarded.
More importantly, you are sending it to the IRS to cover your basis, so even if you don't have your ex-husbands current address and you do send it to his last known address, the IRS still has record that you officially are saying that half of the money was his. This is more for you than for him.
Whether or not you can figure out where to send it to him, you would still only need to report half of the 1099-S on your return since only half of it belonged to you.
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