dielectric plate material and circular Feed

Dear all,
i'm using an NSS 703 services at Indonesia Center java,
and we get the carier w a reguler 12 f mesh prime focus antenna, reguler feed w servo and c-band Norsat LNB 3xxx series.

this service is used for DVB Ip RX/downstream only services,  w 45000 Kbaud,  and RHCP polarity.

so far we put a dielectric plate on the feed  for change to Circular Pol. but now we dificult to find this material of Dielectric plate here.

can  anyone here give some advice, what kind of material that easy can find for replace with no reduce of quality.

and did we can made a simple c-band circular feed ?
any advice ?

thanks and regards

Boni-