I would recommend ringing the shops beforehand to check if they have your size/style you want in store as it could save a wasted trip! I ended up buying mine online too cos I couldn’t find the size I needed in local shops but some shops you can order from their website and pick up for free in store e.g. Debenhams, John Lewis, M+S. You could try either a bigger cup or back size depending on what is tight and see if it feels any better?
I found the sports bras quite uncomfortable at first as I’ve never worn one before and I found it felt tight and restrictive but I got used to it after a while, especially when I felt less tender. x