Hi,
I did a quick search for you and it seems you CAN get a visa at this crossing. I looked
here and found this:
Xa Xia, Vietnam/Prek Chak, Cambodia (Ha Tien crossing)
Cambodian visas available on arrival at the border. Vietnamese visas NOT available at the border. (You must get your Vietnam visa in advance.) There are conflicting reports about whether Cambodian E-visas are accepted.
Open 07:00-18:00
This page confirms they are available, apparently for $25 US.
Happy travels!
Laura