Hi. You can get one online. There will not be an option for La Cai. GO through the forms to purchase one anyway. When they email you the proof of the visa for your approval, OK, inform them that the arrival city is incorrect. They will adjust it along with the price. It will be closer to $50 for a one month visa. I've done this last spring and will be getting one again.
If not, the Vietnamese consulate is not that far from the train station.