Frosty is half dead....

My only Christmas decoration is failing. He has only been lit up about 3 weeks and a bunch of his bulbs are now not burning. I tried using the extra bulbs that came with him, but have been unable to get him all lit up. I finally went and bought a little string of white lights that I was just going to add to him, but when I got home I read on the box that they are inside lights. I can't find replacement bulbs for the originals. The directions say to replace only with 3.5 v bulbs. The only ones I can find are 2.5!