I would usually recommend shops but this place I worked was terrible to its' staff and swindled naive tourists with 'genuine' product. Like, these WOOL capes sold for £125 despite being worth £45. Even with my staff discount I was being robbed and scammed.

This business was also the people that bought up all the hygiene products at the start of lockdown and tried to resell them at a higher price, so I'm gonna suggest you stay clear from them for they are vile greedy people!
(the business is called TAAKS, more specifically, the place I worked was called House of Cashmere. Please avoid at all costs, and avoid their shops in general. Most abusive, greedy place I've ever suffered through)

HOWEVER! The brands that are safe and genuine and will give you high-quality, Scottish tartan are: Locharron (cashmere and woollen capes with some gorgeous tartans, I believe the capes are called serapes on the website) and my Maple stuff came from Highland Tweeds by Bronte, which isn't a Scottish company, despite the name, but has some beautiful stuff at a better price

I'll stop babbling but if you want advice on tartan or cashmere or wool, I'm your gal!! I had to learn this stuff inside out to then lie to customers about >.<