Acrylic polymers like PMMA are transparent and reasonably flexible.

In addition to PMMA, a number of polysiloxanes and polycarbonates are transparent and some of them with good mechanical parameters. You could select the suitable polymer on the base of the exact value of the flexural and tensile stresses. In addition these parameters coud be adjusted to your requirements by modification of the basic polymer.

@Todorka

Thank you for your answer.

Unfortunately I don't have a so deep knowledge in chemistry and polymers science, so what I need is a commercial material name or an indication about a company I could talk with for the production of the belt I need.

Dear Flavio,

you should try here: http://corporate.evonik.com/en/products/products-in-the-web/pages/default.aspx