Shop Local - Northern Scotland

Not aware of any Farmshops but there is a branch of Harrods at the Falls of Shin ! And don't forget the Black Isle Brewery - a couple of miles north of Inveness - for the best beer in Scotland.

Tesco or other supermarket chains (only in sizeabe towns and cities) are your best bet for stocking up on food supplies. There are small corner shops and grocery stores with a small selection of fruit and veg in villages, often attached to petrol stations, but choice is very limited and prices higher. OK for milk, bread, apples and spuds, but for anything more substantial go to a town for more choice.
Because of the climate, most farms are dairy, cattle or sheep, so most fruit and veg have to be brought from outside the area or imported, so no farm shops as they are known in England.

Depends what time of year you are travelling, but in Aug / Sept, you have a good chance of finding blackberries to pick in Scotland.
I suspect that the self catering accommodation landlords have advised you to stock up at Tescos because it's relatively cheap and has a wide range of food. There are also Aldi supermarkets in Inverness and Elgin and Lidl supermarkets in Inverness, Thurso and Wick - both are discount supermarkets, with limited ranges, but very good value and good quality, although they don't take credit cards, just cash or debit cards.

If you can afford the extra I'd say buy local to where you are staying if its at all possible. That way you are putting money into the local economy rather than into the gaping jaws of TESCO.