The parts manual doesn't list the rubber feet separately so, they probably come on the case from Acer's supplier.
If you can't find the OEM part, Ebay has a lot of other feet and generic feet you might be able to adapt.
As well as Amazon
https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=laptop+rubber+feet
