You can buy bits of clear plastic with anti-reflective coatings from Edmund Optics, at prices which might be reasonable for the consumer if you really love that frame. For example, Anti-Reflection (AR) Coated Plastic Windows:
Our Anti-Reflection (AR) Coated Plastic Windows are a highly resistant, lightweight alternative to glass windows. Combining durability and high transmission, these windows are an ideal cost-effective solution for harsh environments and display protection. Optimized for both indoor and outdoor use, the Anti-Reflection (AR) Coated Plastic Windows are laminated with an AR coating to reduce glare and reflection while transmitting a high level of visible light.
A piece measuring 8"×10" is listed for $75.
This is, however, 0.05" (a little over a millimeter) thick, which may not work in the frame. Additionally, I'd make no guesses at all as to its properties for archival preservation, so consider that if the print you want to display is valuable. Your best bet might be to take your frame to a local framing store and describe your needs.