Most of us put the familiar wire cages around our tomato plants when we transplant them into the garden. They need protection for a while, from the wind, especially, until their root systems are well-established.
You can use old plastic bags, but they usually rip in a day or two. You can wrap pieces of plastic around them, but that's a pain, and they don't stay on very well.
OR... you can buy one of my Kerry-All tomato cage covers. Made of strong reusable woven polyethylene, they are silver on one side and black on the other. The plastic is coated on both sides with an ultraviolet inhibitor, so they will last for years in the sun.
They fit the typical wire tomato cage (with three rings) with enough material to fold over the top ring and secure with a few clothespins. You can raise the cover up a bit to let air flow under the cover on really warm days.
The $7.00 price (for two) includes postage to anywhere in Canada or the U.S. E-mail about quantity discounts.