NJOY pure plugs are very nice. High quality material, goes in easy, stays in place. I would look into one of those. They come in 4 different sizes, so you should be able to find one that will work with your body.

My favorite is also Njoy. I have the Pure Plug Medium and it's really easy to insert, comfortable to wear, and very stimulating. My other favorites are the Jollies Luna and the Tantus Ryder, but they're both significantly larger than the plug you have now. The silicone beginner's kit is worth trying -- with three different shapes, you're bound to like at least one of them.