I've bought things from Motherhood Maternity (they have a section in the store to try things on) and from Old Navy (on-line). I loved the things I got from Motherhood Maternity, they are very comfy and more true to size. The Old Navy stuff seemed to run large.
Re: Plus Size Maternity
Mostly just online.
https://www.motherhood.com/maternity/plus-size-maternity-tops.asp
https://www.jcpenney.com/jcp/XGN.aspx?DeptID=70656&CatID=72501&shopperType=G&N=486&SelDim=29~&Ne=
https://www.target.com/Maternity-Clothing-Womens/b/ref=nav_t_spc_1_6?ie=UTF8&node=1293426011 ( just look for extended sizes)
https://www.plusmommaternity.com/
Then depending on your budget this one is a bit more expensive.
https://www.bellydancematernity.com/Tees-Tanks-c-219.html
These are the only one's I've found so far.
Motherhood/Destination Maternity has a small section in-store. I think there was one rounder display in the store that I visited.
I'm borderline regular/plus and found that the regular sizes fit well and some were even too big.