It depends on if you are going to trace around the template with a pencil or use rotary cutter against your template. If you are going to trace with a pencil then the clear cutting mats from the $ store work well but you can use a cereal box cardboard, the lid from a margarine container, if it's big enough or sometimes the clear hard plastic packaging from a product. Here is a picture of the $ store mat.
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If you plan on cutting against your template with your rotary cutter, then some hardware stores will cut a simple shape for you from thick acrylic.