Hi Bill,
You can buy provisions in Inverie but it's best to have a flexible menu plan, or bring with you what you need!
Finding water is not usually a problem in the highlands, and it's relatively free from bugs such as giardia etc. The main contaminants tend to be other campers with poor water hygiene.
Sourlies and Barrisdale can get stupidly busy. As Mike says everyone and his dog goes to Knoydart for a wilderness experience so it can sometimes feel rather suburban. Yes there are other bothies in the area - in Glen Pean, Glen Dessary, and Oban at the head of Loch Morar for instance. They can also get busy. If you're planning to make directly from Glenfinnan to Sourlies be aware that it's quite a demanding day - cutting across the grain of the terrain with much rough ground and a lot of up and down!
Washing up? Fine gravel from the stream bed, sand, or a heather pan scrub, and a good rinse usually does the trick. Make sure you use a biodegradable soap and dispose of waste water carefully though.
[
rant mode on]Knoydart like many popular camping areas suffers from an overload of used loo roll. your average quilted 2-ply takes a ridiculously long time to disappear, I have to spend the summer wading through the stuff, so please make sure you burn it or pack it out rather than bung it under a stone
. Absolutely worst for this are wet wipes![
rant mode off]
Getting the boat to Tarbert and walking out along Loch Morar is a good option, very scenic and quite gentle. East of Tarbert the shore of Loch Nevis is pretty full-on and trackless.
Be aware there are lots of deer ticks in Knoydart. There is a small incidence of Lyme's disease with infected tick bites, although allegedly it's usually later in the summer that the risk of lyme's is highest (don't ask me why - something to do with the life cycle of the tick)
Watch out for TitaniumDude, he has a habit of chasing deer around stark naked in the middle of the night around here.
After all that, have fun Bill, it's a grand place!!