I’ve bought from various sellers on eBay and Etsy, Secret gardening club, J Bowers, Thompson and Morgan, Hayloft, Sarah Raven, YouGarden and Ashwood nurseries.
I also get a lot from Morrisons, Aldi, Poundland and B&M though those aren’t online.
I would not recommend J Parker’s as they delivered me a dead as a dodo completely brown and crunchy
I have heard many similar experiences about them.
Thompson and Morgan are fine, slow delivery, plants themselves are healthy enough but they are way, way over priced imo.
My seeds from Sarah Raven never germinated so I haven’t used her again.
Ashwood nurseries sent me the wrong plant.
I’m not sure who was at fault, whether it was them or the breeder or whoever else they bought the plant from, it was labelled as the plant I was after but it turned out to be a totally different cultivar.