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Actually they don't need to be too fancy. I preferred a Fahrenheit one myself because of increased specificity (smaller gradient) but it doesn't really matter. I bought one like this one which worked perfectly well.
I have been using one I got from Amazon for £4.99. Can't compare it to anything else as it's the only one I have used....does the job perfectly (well except to make me pregnant ) Uses Celsius which I perfer.