There is a Newdays convenience store under Maihama station (the rail station for TDR), open from 6:30am until 11:30pm. See
http://shop-search.j-retail.jp/spot/detail?code=0000000404.
Unless you have a reason to prefer a Lawson or FamilyMart, the closest branches are considerably further away (you might walk to them from the Ambassador Hotel, or perhaps even the Mira Costa, but it would be quite a trek from the
Disneyland Hotel). See
http://store.lawson.co.jp/store/212890/ and
http://as.chizumaru.com/famima/deta...28&y=128202.109&lscl=8424&lrot=0&pgret=1&c2=1. There is a 7-Eleven north of Maihama station, but there isn't an easy way to actually get there. See
http://www.e-map.ne.jp/p/711map/dtl...1&&his=ar,nm&srchplace=35.6507139,139.9054611.
Ikspiari has a mini-supermarket in the basement, although note that it won’t be open at breakfast (the hours are 10am-11pm). See
http://www.ikspiari.com/en/shop/1109/index.html.
The Tokyo Disneyland Hotel does have a reasonable selection of food items in its shop, and the prices didn’t fell outrageous. See
http://www.disneyhotels.jp/tdh/facility/gift.html.