Namibia has well-stocked grocery stores in all the major cities and larger towns, but big towns are few and far between so stock up whenever you can. Local meat is excellent and you’ll often find exotic venison for sale, such as springbok, eland or kudu. These usually come from nearby game farms and ranches, and the quality is generally very good.
Unless you’re staying at a very high-end lodge, don’t expect much gourmet dining. There are some good restaurants in Windhoek and Swakopmund and eating out is not expensive by European or North American standards. Tipping is not mandatory, but 10% is typical if the service has been good.