Here is a link to where melodicas are on Buyee. It is under Musical Instruments—Keyboars—Other.
When you get there you can scroll through the pages for the most recent items that are being auctioned.
eBay is the other big source, though sellers may not be willing to ship to Vietnam.
I have bought many used melodicas from both Buyee and eBay.
That would be P-37D, not E. The usual cost is about $100 U.S., sometimes with free shipping to the United States from Japan. The price would be lower sometimes if bought directly from Japan, but then there would be shipping fees. But if melodicas are now being sold in Vietnam, buying there might be best. For used melodicas, though, Buyee or another Japanese auction is best for Japanese melodicas.