Local avaliability of Controller required for RTOS

Hello all,

So Far i had done development on single threaded applications using
PIC 18F2585 Controller,

Now i want to jump to Multi threaded applications ,

for this please suggest which controller i can purchase and which RTOS should i install in it.

as i know , for RTOS option avaliable are
RTOS LINUX and Micro C OS

and controllers : raspberry pi or beagleboard or arduino Kit

but which RTOS will be supporting which Controller that i do not know,
i hope so you got my query, please suggest your thoughts and guide me for my next step.

Thank you
SANJYOT (BANGALORE ,INDIA)