My friend bought Paralympic Athletics tickets during the first week of the main games, but they sold millions that week and they're all gone now. Keep an eye out though as they'll sell some off each morning as they come back from the different countries.
Nevertheless, when I went, I sat opposite a block of about 150 empty seats. When I asked about them, they said they were allocated to international guests, none of whom appeared to have turned up!
I suppose they don't know they're not turning up until the event had started and no-one was there. All other blockas were packed to the rafters.