بسم الله الرحمن الرحيم
The best store for me is www.sunnahstyle.com
Their abayas are large and plain .
Also sister check their blog and you will see there is something called factory seconds section ,where they put some abayas that have little defects for very cheap price .You can find a $50 abayas there sometimes for $15 ...
They have nice and large Hijabs also ..
Got to second this. I've got a butterfly abaya of theirs and it is great! Being hippy and plus-sized, I find it hard to find anything that fits and with SunnahStyle pretty much their entire range would fit me off-the-peg. I'd suggest looking at the 'value' range which is designed to be cheap, but not low quality.
And yes, the factory seconds are also worth a try. It's often tiny minor things such as wonky stitching or a slightly different material, and there's massive discounts. Plus they include seconds hijabs, shaylas and niqab in there too.