This will depends on the look you’re after. If you like a soft cushion look, you should choose the same size insert as the cover.
For a plump and filled out look, you should get an insert that is one size bigger. Here are our recommendations:
Cushion Cover Size Insert Recommendation for a plump look
30x50cm 30x50cm*
45x45cm 50x50cm
60x60cm 65x65cm
70x70cm 75x75cm
*use same size inserts in small cushions
Due to shipping cost, we recommend buying your inserts locally. Here are some places you can check out:
Ebay – plenty of local sellers and sizes available
Sportlight
Lincraft
IKEA – has some great value feather inserts