Ranma ½ is getting to be an old series, and due to this finding certain products is becoming more difficult. Here are a few places you might want to check out.
Rightstuf has some of the best deals on anime and manga on the web, so they're always a great source to help fill the hole in your collection.
Viz is the English publisher of all of Takahashi's manga series, and the large majority of her anime as well. Definetly give them a gander.
Depending on when you check, you can almost always find something interesting on eBay, especially rarer items.
One of the older online stores that specialized in anime and manga, they don't have a huge selection of Ranma ½ goods, but you might find a few things here.
A chain of Japanese bookstores that will do special orders, they're a great source for getting Shonen Sunday and other weekly magazines.
A great store based out of Japan, the brick and mortar store in Osaka has one of the most incredible selections of anime and manga products I've ever seen. The online catalog isn't too shabby either.
One of the largest North American online stores for anime. They have a very good selection of DVDs, books and CDs.