You don’t need any 1099 substitute if you report your foreign social security pension in Form 1040 “line 8 Other income from Schedule 1 line 10”, and obviously also in such “Schedule in line 8z”.
Should IRS require upon the tax return filing anything else, you can provide either the receipts issued by your country Social Security Administration or your US banking stub if you received the money here in the US. Both are valid .
