I've done a couple of mascot costumes, though I only got into making ones with actual heads and such last year. You'd be surprised what you can make with just some general craft supplies and creativity--I had no experience at all last year when I started my Akamaru, and I think it turned out pretty good.
Last year, I used the following things:
- A plastic face mask
- A small amount of metal mesh
- A styrofoam cone
- Finishing glaze
- Fake fur
I started by cutting off the bottom half of the plastic face mask and adding the metal mesh to the bottom of the mask, sculpting it to fit my face. I then covered the mesh with a scrap of an old shirt so the ends of the mesh didn't poke me. I used a hot glue gun to affix the shirt piece to the mesh and the mesh piece to the mask.
I then cut the styrofoam cone so it fit against the nose/mouth area of the mask. I hot glue gunned it there, and then went over the cone and mask with Celluclay until I was satisfied with the shape of the nose and mask. I went over that with the finishing glaze, and added fur from there.
Or you could use Wonderflex, which I just began using this year on my in-progress costume. This was a great experience, and I highly recommend it. The stuff is very durable and easy to work with. It's a little more expensive than the other option, though, so be warned. And be sure if you're going to use Wonderflex to sculpt a head, invest in a styrofoam head to use as your base.
If you have any more questions or want more in-depth descriptions of how to make it, feel free to PM me!
I wish you the best of luck!