Had exactly the same issue when I put in two pendent lights. Buying a whole board for a 3" hole seemed excessive. On the way to the hospital I saw a house refurb with a load of plaster in the skip, so nabbed some of that.
My suggestion would be;
1) go for a ride at lunchtime. There must be tonnes of people having work around you.
2) go to a wickes / similar and ask if they've got any spare broken bits.
Had exactly the same issue when I put in two pendent lights. Buying a whole board for a 3" hole seemed excessive. On the way to the hospital I saw a house refurb with a load of plaster in the skip, so nabbed some of that.
My suggestion would be;
1) go for a ride at lunchtime. There must be tonnes of people having work around you.
2) go to a wickes / similar and ask if they've got any spare broken bits.