Yes, that site shows a wide range of seeds - not sure I would count all as being a rock garden plants in that section though. Also I notice that there is a minimum order of $64.29 - which seems a lot to me.
There is a website which specialises in plants for shade :
http://www.plantsforshade.co.uk/ and I think that you could be guided by the sort of plants they advise for shady conditions. (They don't sell seed and they are closed for plants meantime - but you will see a lot of plants suitable for shade listed there. )
See also this site to get some ideas :
https://www.cleevenursery.co.uk/alpine-rockery-plants/