As a fellow plus sizer, it is hard. And it's harder the bigger you are. At a size 18 I had a fair few options, at 22 I make do.
How many inches are you under the boobs? About that will be your back size.
Bravissimo do some bras up to a 40 back. Mine was feeling a bit tight so I ordered a 42 online (Amazon) in a few cups sizes to try. Would a 40 fit you? If so go there. I like the panache tango super bra - great side support snd bravissimo are great
Rigby and Pellar have always been good at fitting. But they had ONE bra in my size last time I was there (40J) I would go get measured there and then if they don't have your size go home and do a big online order and try lots of bras. SimplyBe have lots of bigger bras.
My answer is losing weight (stone and a half down already this year hurrah) I miss being a 36 and being able to get nice stuff
The best are definitely Prima Donna. Pricy but worth every penny and look pretty. You can buy online but I think it's really important to get a fitting first. I go to Rigby and Peller once a year to check my size is still right. Really recommend them if you can get to one.
I used to be a 42g and could easily buy bras at m&s, simply yours, yours clothing and if I had some money then the branded ones like panache and triumph.
M&s definitely do a 44c. I just googled them. Someone will be along to say you probably aren't a c cup. I think c would imply quite small boobs? (3-4 cm bigger full bust than back could have that totally wrong though)
Measure yourself using the guise above and then go to m&s and pick up an assortment if bras in that size and try them on. Full cup, balcony, padded, youthful lift etc.
PS would definitely recommend a Prima Donna or similar from R and P (or, once you've been measured online) rather than any M and S bras.
A bra is one of the few things - haircut and glasses are two others - that are always with you whatever you wear, so it's really worth investing as much as you can in that. Even more so for those of us with big busts who want to avoid the pillow/monobosom effect, I'd say.