Remove the drive belt:
Use a 14mm socket to push the tensioner away from the belt by turning it clockwise. You will need to use a cheater bar to move the tensioner back.

Mazda part # = PE0315980B for the belt tensioner

Try these on-line resources for the replacement parts;
www.autopartswarehouse.com, www.partsgeek.com, www.rockauto.com, www.partstrain.com, www.jimellismazdaparts.com, www.autoanything.com, www.newautoparts.com - to list only a few.

For example, they (New Auto Parts) list the drive belt tensioner from around $97.00 for your vehicle.