k2aqua is the seller I use - all their plants are shrimps safe and I have shrimps and nerites. They have been getting more plants, and more less common plants recently (if that makes sense)
I've also bought the odd plant from Pro Shrimp, again because they are shrimp safe.
I've been looking at another shop though I haven't bought from them yet. They source their plants from Tropica which is in the EU so they are also shrimp safe.
Aquarium Plants Online UK - UK's leading online supplier of Aquatic Plants. Aquarium live plants & supplies. Delivery to UK, Ire & Europe
java-plants.com
I know that other members use Aqua Essentials but as they don't say if their plants are shrimp safe I've never dared to use them. If you don't have shrimps or snails, it doesn't matter.
[Plants grown outside the EU have to be treated with a snail killer and this still applies to the UK. Plants grown within the EU don't]