So I've been struggling to find a bra that fits me well, I'm a 42b but have a hell of a time finding bras with wide straps and backs in this size
Any brands or places anyone can think of?

I’ve sent more traditional bras back, than I’ve kept. I can’t find a normal that fits, at all.

I’ve had success with these two styles. The first link sports bra is sold under multiple name (brands) on Amazon and eBay, but they probably all come out of the same factory.

https://www.amazon.com/Womens-Comfort-Workout-Low-Impact-Activity/dp/B07G5VNVG8

https://www.amazon.com/Carole-Martin-Full-Freedom-Closure-Wireless/dp/B07CNW7CVZ

Best of Luck to you.

I have the best fit with Elomi and Godess. I shop at a lo al place but they ship and have a good return policy. 
https://www.lovelbb.com/

Give it a try. Ginger is the owner and she is great, you can call and she will tell you what they have in stock and what she can order for you.

I like the fruit of the loom on Amazon