What kind of door is it? paneled, flush? If paneled, the best door is 1 3/4" thick routed cellular PVC with a perimeter sound seal and a drop seal at the floor. Expensive but very sound resistant. Make sure a door undercut is not required for air return for a forced air HVAC system.

I think the best and the cheapest available option would be a standard exterior door having gaskets around the edges, the seal at the threshold. It is 10x heavier than a standard interior hollow core door...you will notice a difference. Check them online, you will get various deals associated too.