You can easily make your own. The foam is an excellent insulator, for maximum effectiveness. A 3 opening angle should allow a 3 unvignetted field of view. As a rule of thumb, a dewcap should be at least 1 times as long as the aperture is wide. A side benefit is that the cap also cuts down on stray light getting into the telescope.
Eyepieces too are prone to dewing.
Warm radiation from your face slows the process, but humidity from your eyeball and breath speeds it up. A tall rubber eyecup, the kind that extends above the eye lens all around, not only blocks stray light while you're observing but acts as a miniature dewcap when you're looking away. The same principle works on large scales. The tree is a giant dewcap, and it can work for you too. If you'll be looking at only one part of the sky, it's nice to have trees around and behind you.