I had this same issue last year. I wanted to use points but no availability. So what I did was reserve a room, converted some of my marriott points to marriott dollars (they had a special discounted rate promotion at the time) so I think for 125,000 points, got $1000 marriott dollars and paid for the room with marriott dollars. So in the end still used points and no money out of my pocket.

By the way, doing it this way, you still earn Marriott Reward points on your stay.