These are just some of the places I like to shop, and I'll come along and add new brands as I find out about them. *AL = affiliate link. If you shop from this link I will make a small commission. (Usually less than £10 per month in total).
Yours Clothing - *AL a great all rounder with most styles up to a size 32 and some larger.
Society+ - I love their tutus! US based.
ASOS - *AL size issues abound in the Curve line, but they have lots of choice and their sales are great.
Navabi - a bit of the luxe end, but quality items and their sales are great.
Elvi - another slightly spendier line, but quality items
Ready To Stare - great slogan tees and accessories. US based.
Smart Glamour - fully customisable clothing from sizes XXS-6XL. US based.
Witch Worldwide - gothic, witchy items and great accessories. US based.
Feminine Funk - great slogan tees. US based.
Modcloth - does up to 4X. US based.
Lovedrobe - sizes 16-32. A great all rounder with casual and glam styles.
Evans - *AL a great all rounder with most items up to a size 32.
Boohoo - *AL their plus line is affordable. Goes up to a size 24 but does many oversized or stretchy items to fit bigger sizes.
House of Fraser - *AL great plus size lines and accessories.
Curvissa - a great all rounder.
Dorothy Perkins - has a curve range up to a size 28 and good accessories.
Interrobang - does clothing in sizes 6-34.
New Look - *AL the Inspire section is a great all rounder with most styles up to a size 32UK, and their wide fit shoe range is second to none.
La Redoute - Some plus size lines.
Missguided - does a plus size range up to size 24.
Simply Be - *AL a good all rounder with many styles up to a size 32 (but not so much in their celebrity ranges). Some styles larger.
Very.co.uk - some great staples in their So Fabulous plus size line, their wide fit shoes are great and their South line goes up to a size 24 but items with stretch will fit bigger. My favourite leggings are the So Fabulous twin pack of one full length/one cropped leggings. They wash so well.
Big Bloomers Company - great plus size underwear and hosiery.
Donatella's - great plus size hosiery.
Places I haven't shopped at yet but intend to:
Chubby Cartwheels - I want everything! US based.
Flaws of Couture - great slogan tees. US based.
LucyAlice - workwear up to size 24