Found this posted by one of the members in our facebook group sharing this great source for PEI sheets suitable for your Prusa i3 MK2.
As we all know, Prusa is very busy keeping up with demand these days and sometimes getting replacement sheets of PEI from them can be difficult because of low inventory.
Here is a great company especially for those in the US where you can purchase all sizes of PEI that can be used on your MK2
The company is named CS Hyde and their website is located here: http://catalog.cshyde.com/item/ultem-pei/ultem-pei/36-3a-3d-11-81-11-81×11-81
Some specifics about their Materials.
Ultem®/PEI (polyetherimide) has struck the 3D printing world as a high performance material for 3D printing beds. Ultem®/PEI film enables 3D printing enthusiasts to produce both ABS and PLA prints. This film offers extreme heat resistance, coupled with high mechanical strength, stiffness, UV stability, and broad chemical resistance. This combination of properties allows this film to be utilized in a wide variety of demanding new design concepts including aerospace,
automotive, and even medical applications. PEI is a functional product for 3D printing due to its extremely smooth finish, high temperature resistance, and ability to provide print placement security while releasing prints smoothly. Ultem/PEI is a “no surface prep” material which allows for no major surface modifications like applying special adhesives before each print. (If possible) Recalibrate your machine after install.
(Similar to PEEK, Ultem® can be considered a more cost effective solution if continuous temperatures are below 338°F). Ultem®/PEI has a UL94 flame resistance rating of VTM-0, is FDA compliant, EU Food Contact Compliant, and ISO10993 compliant in natural color.