Try also contacting Hans Ola Ericsson. He was Messiaen's last student and I gather he became quite close to both Olivier and Yvonne. I met Ericsson at a masterclass he was conducting on Messiaen and managed to speak with him privately afterwards. I also received a copy of one of OM's last pieces which, at the time, was unpublished (not sure if it has since been published however).
He's certainly still under copyright as CT pointed out so any scores available "freely" on the net would not be legal (along with a whole lot of other stuff available on the net).
Sorry but it has evaded my addled memory at the moment. I'll try to dig through my scores sometime soon.
One of my favourite early OM pieces (apart from the beautiful "Le Banquet Celeste") is the "Diptyque". First part is quite challenging but the second part is just wonderful, easy to float away with (and it was that part that he re-arranged some years later for the "Quartet for the End of Time")