Question about walking boots, insoles and arch support

[Icelandholiday]

My trusty old walking boots finally broke after many years, so I went to buy some new ones. I spent a whole hour at the shop with a very helpful guy who let me try on loads of pairs. Went out for a muddy walk with my new boots and realised they weren't quite right for me, because I have a slightly flat foot which isn't being supported. My old boots were tighter and held everything in.

Considering these boots were expensive and I can't take them back to the shop as I've worn them, is there some kind of specialist insole with arch support I could add? Preferably not too bulky so they don't take up loads of room inside the boot. But I don't want to have specialist orthotics or anything, just normal arch support if that makes sense.

Or any other ideas how I could rescue the situation?

I'd go back to the shop and ask.

Yes I buy orthotic insoles with a deep heel cup due to achilies tendonitis and also an arch support for my knees to avoid my ankles falling in

The massive issue though is that I have to buy shoes now that I can take the insole out of to fit my special insoles in otherwise the foot is too high up in the shoe or even worse I cant get the shoe on due to the insole

Its a bloody nightmare

Ive now got over 100 pairs of shoes that I probably cant wear.

I use Superfeet insoles. The shops around here that sell walking shoes and boots tend to have some of the Superfeet insoles to try with your boots, which is useful as they're not cheap and there are several different (and confusing, at least for me!) types.

The one that suits me best is the copper one. I have flat feet and some overpronation.