Buceplants and Aquatic Arts are two I have had success with. Don't go for the cheap stem plants on ebay, usually listed under "buy two, get one free", as 80% of mine died.
I have used 2 plant shops only... They are both on eBay and etsy. One is named Marcus's Fish Tank and the other one is Canton Aquatics. Both of these are really good shops that have free shipping (PS: their shops are a tad bit cheaper on eBay.)