www.cheapcycleparts.com But don't expect it to be cheap. I am missing the "Goldwing" emblem on my right sidepanel on my '85, and they still have it. For $30. Not at the top of my list.
There are other ways of replacing stuff like that. I can get an almost exact copy of that emblem from an online graphics place for about $10. I can design and size it myself, and pick my own color. The only difference is it will be a flat vinyl sticker, not a 3-D raised plastic emblem. I have used this method before when restoring both cars and bikes.
In fact, you can even recreate complicated graphics, if you have a picture. I have an illustrator program that you can enter a picture into, even an old faded picture, as long as it if front on, and not at an angle, the computer will measure all the lines and color differences, and create a sharp new image that is so close to the original that in most cases you can't tell the difference. Then you take that file to a sign shop, and they will use it to make a vinyl sticker out of, one of those with the paper backing.
If they are still available OEM, they are about $25 each retail. My dealer discounts them down to around $18. Check with Cyclemax Ohio, http://www.cyclemax.com/. They sell OEM parts as well as aftermarket stuff and their service is top notch. They will need to know the model, ie Interstate, Aspencade or SE, and color of your bike. The emblems are different for different colors and models.