Where to buy plus size clothes over size 20
eShakti - this online retailer makes adorable dresses/skirts/shirts and accessories up to size 26W. In addition to this sizing, they also offer custom sizing for their items (including alterations on sleeves, hem length, bust size, etc).
Forever 21+ - I really love shopping in store as their selection is much larger, but they have online shop too. Their sizing goes up to 3X, though it really varies wildly. Worth checking out for really trendy looks.
Evans UK - this great UK shop goes up to UK size 32. They’ve got lots of trendy looks. Downside - I find international shipping to be problematic with returns. Not specific to Evans UK
As for in-person shops
Lane Bryant goes up to size 28
There are tons more resources out there and FuckYEAHCHUBBYFASHION has a massive list.