Asklemmy

Rather small compared to the deep sea ones you are probably thinking of. They’re terrestrial so many people will call them rolly polys or something like that.

The largest species I currently have are Porcellio hoffmannseggi, as far as I know the largest terrestrial ones and they get to about 5 cm total length and maybe 3 cm body length.
I once tried to breed amphibious Ligia oceanica ones that I collected, commonly referred to as sea slaters. They were a bit bigger in terms of body length but needed saltwater and also didn’t fare well with room temperature I am afraid.

However the P. hofmannseggi are fairly easy to keep and inexpensive. They just need a small container with a bit of soil, leaf litter and some dead or rotting hardwood to hide under, I spray them about once per week. It’s a great hobby.

There are also small freshwater species that can be kept in aquarium but they are not sold anywhere and you can usually collect them yourself.