Go to your local big box hardware store and buy a 2X4. Have them cut it into pieces that match the width of your bench leg + and inch or so (or do it yourself.) If you want to get fancy, buy some shoe molding and nail it to the 2x4's, framing the bottom footprint of the bench. This will keep the bench from slipping off the blocks when you slide it around. To be really sharp, stain the whole thing to match the bench. Total cost will be less than $10 I'd bet!
-OR- Just use old hymnals like I have done in an emergency!!!
Good luck!
Holtkamp/Parkey pipe organ
Galanti Praeludium home instrument