1/8 inch is standard for small frames; 1/4 inch is necessary for large wall mirrors to prevent "funhouse" warping.
They often provide more patient service than big-box stores. where to buy mirror glass cut to size
Many locations offer basic glass cutting. 1/8 inch is standard for small frames; 1/4
Most offer professional measuring and installation services. where to buy mirror glass cut to size
Many carry thicker mirror stock suitable for frames.