How can I find Bottom Rubber Feet Foot For MacBook Pro 15" A1398？
I searched Amazon.com and ebay.com, can not find this model.
Apple actually has an article specifically about this, right here. Apparently they have a rubber foot replacement kit which you can get if you contact them and request it.
Personally I would just find some soft-side velcro, felt, or some other craft material, cut out a few small circles with an xacto-knife, glue it on with Loctite (which is what Apple uses according to their article above) and call it a day.
If it really bugs you and you want someone else to cut out the little circles for you..
As a former Mac Genius, I can tell you it's impossible to buy the feet separately as a replacement. You will have to get the bottom "plate" replaced. If it's under warranty, awesome, if not, it's less than $20 to get.
Look at this eBay link here: http://www.ebay.com/itm/181701854491?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1558.l2649
I hope it helps.