I'm 5'5" and weigh 250 and comfortably use a medium maya. I am pear shaped and carry much of my weight in my hips and butt though. If you're apple shaped or want more leftover fabric for draping (I only have about a foot and a half left over) you might want to go with a large.
I don't know anyplace to get them cheaper when they're new, but I did find one in a Baby consignment shop. I know other people who have found Maya's in 2nd hand shops as well. They're usually in like-new conditon too!