if you are refering to the plastic fold-in, fold-out legs for height adjustment then I believe you are SOL. a quick googling revealed that many people have noted that logitech does not (and does not want to) stock these.
if you want a permanent (but non-folding) solution you can get trapezoidal rubber cubes with adhesive on one side that will stick to darn near anything. i've seen them as low as about 3/6" and up to about 1". i think they are classified as non-slip feet by the local hardware stores.
if you are referring to the square or circular skid plates on the front bottom edge of the keyboard: you might have to go with adhesive rubber (also at your local hardware store) and trim them to fit the spot.
either home-made solution should cost you less than $8