It's currently up for pre-order on Gearbest at $210 for the 4/64GB model, the 6/128GB one is $250. You can reserve a place in line with a $10 deposit, shipments are planned to start between March 31 and April 2 and the online retailer is ready to send the phone worldwide (or it could start shipping in 16 days, there's currently conflicting information).
Gearbest is offering a "deposit expansion" bonus, meaning that $10 deposit will net you $40 in savings (meaning the final price will be $170 or $210, depending on the memory configuration).
You'll find the Redmi Note 9S in Blue, Black and White over at Gearbest.com.
Note that AliExpress is the official Xiaomi partner that will be selling the phone worldwide. It will sell it at $250-$280 for the 4/64GB and 6/128GB versions, respectively, starting on April 7. However, a two-day flash sale (for the Mi Fan Festival) will see the prices drop to $200 and $240 on April 7 and 8 only. Also, there will be 2,020 units of the MFF 2020 Limited Edition (with the MFF logo on the back).
The 9S will also be available through Lazada and Shoppee in Malaysia (for MYR 800-900). In Thailand, it will sell through Lazada, Shoppee and JD for THB 6,500-7,500. It will also be available online in Singapore for SGD 300-350.