I've learnt in the past Clark Rubber is not always the best foam after buying high density stuff to cover a chair base with. Far from quality and far from cheap. They used to be the go-to place, not so sure now.
Though I would search online for decent quality stuff and I believe you can buy packs of good quality plucking foam to fit certain cases or cut it down as required, straight edge and very sharp Stanley knife allowing for errors with applied pressure squashing the foam.
I guess Clark Rubber gives you a starting point if you have one near.
https://www.caseline.com.au/custom-pro-inserts/
https://www.intricut.com.au/
https://www.thefoambooth.com.au/custom-case-inserts/
I am not recommending these companies, a quick search online came up with these companies in the results, as to quality, maybe check for reviews.