You may order replacement feet online. Go to https://account.microsoft.com/devices and register your device. After it is registered, select your device, select start service online and choose Accessory > Misc. Enter the description of the problem and your shipping address. Then pick the color of the feet to match your device.
If you still need help, please contact Surface support. An advocate will be able to assist and send you replacements for the missing feet.
Hope that helps,
Greg