If you're thinking small (25 to 75 or 100 points), those don't seem to become available on the resale market as frequently, and the closing costs really seem to increase the total price disproportionately. There's a waiting list for add-ons through Disney. Just contact your guide or MS and asked to be put on the wait list. They won't predict how long it might take, but you can ask how far down the list you are. I've read that some people have been waiting a few months and others a year. Disney will only sell you points for your use year. I know that some became available about 10 days ago, because our guide contacted us (we didn't take them because we were waiting for VWL).
If you're looking for a larger add- on, you might get a better price and get quicker results with a resale. Plus, you can buy any use year you want, but you have to then manage the different use years. Good luck!