do you have a reference for the stone cobbles?
The brick like pavers do a similar job to cobbles at a fraction of the price - same principle. But you do have to dig down far enough for a bed of sand and ideally hardcore base. Itâs very good at minimising mould/algae but itâs not the simplest installation.
If you have space you could lay something like slate or outdoor tiles - but with the cement you will lift the surface by an inch or so. Iâd drill through the slab so it drains first. If youâve space you can also fix things like the fall so it always drains.
There are products that clean and help stop mould/algae or more accurately reduce itâs regrowth - but most things will have some mould/algae if they,re damp and shaded.
Ours came from
But I have found some other sources that I think might suit @SuePJ
https://www.noblema-cobblestone.com/reclaimed-setts
This last, rather endearingly, state they speak English
A nice range of terracotta tiles here: