Any other resort. However, unless you buy 150 or more points and put it under another master contract, they will only let you add-on using the same Use Year as your master. Therefore, if you want one of the sold out resorts you may have to sit on a waiting list for weeks or months while they accumulate the points that match your UY.
There is a minimum of 25 pts for any add-on. If you want to finance the purchase, you need to add at least 50. And there are also minimum thresholds to take advantage of discount pricing. For example, the current $86 price for SSR points requires an add-on of at least 115 pts. If you buy less than that, I believe the price is the published rate of $101 each. Typically no discounts are offered for the other sold-out resorts.