You have a number of options, actually. Certainly calling TTS is a great place to start. Depending on which resort you want to add points for, resale may save you some money. At some resorts, it could save you a lot of money, depending on the size of the contract.
However, since you are already an owner, you also have the option of adding on directly from Disney. Sold out resorts are $89 per point and SSR is currently $87, although it is going up June 19.
You can add points through Disney in increments as low as 25 points (50 if you want to use their financing), and points are pretty much available at most resorts. BCV is tough for some use years, but most of the add-ons are pretty easy. You may have to waitlist for a short time, but nowhere as long as you will wait for ROFR, closing, etc. with resale. And of course, there are no closing costs buying direct.
If you are thinking of adding 100 points or so, you may want to take a close look at the SSR buy 120, get 100 free one-time points and no 2005 dues incentive. You can rent those 100 free points for $10 per and drop your cost to below $80 per point.