Foam: What Kind, How Much?

I know that there has been at least one post about this in the archives, but none are specific enough.

1. What kind (how thick) of foam should I buy? (I am aware that it will vary for the ears, muzzle, face, and paws)
2. How much foam should I buy? I will be building off of a balaclava base, so the head will not be completely made out of foam. The handpaws will be stuffed (or "puffy" without looking "toony"). The feetpaws will be plantigrade.
3. Where should I buy the foam? I don't want to go with "el-cheapo", low-quality foam, but I don't want to go broke either.

Any help would be appreciated.