While it's allowed, at least in most parts, to wild camp in Scotland, I prefer to have a campsite to arrive at in the evening. The density of campsites in Scotland is definitely less than for example in France, nevertheless, it's quite doable to find one for the night.
Another option which seems to be quite common is the possibility of camping at the parking ground of a pub. At least when you have a meal or pint there. This has the advantage that you might be able to use the facilities available and that you get some food. I did this at the Dundonnel Hotel and their staff was super friendly and helpful! To find out whether this is an option I would recommend checking the Google reviews of a site, it's usually mentioned there.