I have accumulated quite few plastic dome cloches over the last few years. They are not particularly expensive to buy and in my opinion represent good value for money. For instance Greenfingers.com are currently selling the Botanico ones at £3.99 for 3 Medium-sized ones, or £7.99 for 3 Large-sized ones. [Some of the images illustrating their products are the ones I submitted last year!]
Cloches are of course primarily intended to protect plants from the weather. They will shield a plant from wind, hail and heavy rain, and prevent frost settling directly on the plant, but just be aware that they don't raise the temperature. You may get a slightly higher temperature inside the cloche (particularly when the sun is shining), but only slightly. I think the wind-shielding effect is the most valuable aspect because when a plant is buffeted by wind it responds by building up its own strength - like a human being exercising in the gym! This can make the plant tougher, which is not necessarily desirable if you are eventually going to eat it.
In addition to shielding plants from the weather they can also serve to protect plants from animals and birds. Look at this poor Landcress plant which has not had the benefit of a cloche. The pigeons have decimated it!
On the other hand, here is a Landcress plant covered with a cloche (note the ice):
Removing the cloche (briefly) reveals this - a much happier plant:
I had some spare cloches in the garage, so I have now got them out and put them to use:
Sorry, pigeons, but salad is OFF.