I get mine on prescription but I have lupus. I’m pretty sure a relatively healthy person who has a vitamin D deficiency would be told to buy supplements. I take Fultium D3 which is prescription only. Your doctor will recommend a specific type if you need it, whether OTC or on prescription.
There was some research released just yesterday showing that basically everyone in the northern hemisphere would benefit by taking vitamin D + calcium. It massively reduces the rate of heart attacks for some reason. Do check with your doctor before taking anything that hasn’t been prescribed though.