This place is fab. I've read so much and it's amazing. You have everything covered

I live in the UK and got my guys from a mistaken order by our local fish shop. So far they have a tank, food, heater, water, etc. Lots of stuff coming in the post too. I'll keep on reading all the info on here and get it all as perfect as I can for the little critters.
But shells are a really challenge to buy. I don't really know what I'm looking at and a lot of the stuff I see on ebay or wherever just say it's a mixed bag of shells which is super unhelpful.
My hermits are only little. Their shell opening is about 10mm (not sure how else to describe their size).
Does anyone have a particular search they use? Or a website? I will buy from outside the UK if I have to. So far I've found no-one suppling from here.
Also, if I was to pick ones up from the beach, would that be OK?
Thank you for all the help so far
