During my skirmish with cancer there have been various websites, books, organisations, companies etc that have 'come in handy' for a whole lot of stuff - from explaining medical terminology to supplying a purple velvet hat.
Websites:

For practical, accurate cancer info

Macmillan Cancer Support
Breast Cancer Care
NHS Choices
Cancer Research UK

For info on obtaining a Medical Exemption Certificate

NHSBSA

Hats/scarves:

Lojangle
Bohemia

Bras:
I'm a 40AAA, a size which just doesn't crop up in the standard mastectomy bra catalogues. I needed a correctly fitting bra before I could be fitted for my prosthesis and was getting quite upset as I couldn't find one - until my husband typed 'where can I buy unusual sized bras' into a search engine! The first couple of sites listed would probably have been 'educational', but then we found Dainty Lady. Their (really pretty) ranges run from 28AAA to 44B and, as they make a lot of items to order, they can potentially add depth to bra fronts or wider straps. Their bras are unpocketed, but you can buy pockets from the standard mastectomy cataloges (I use Amoena) and sew them in yourself, or send bras off for pocketing.

Other useful tips:

Wearing very dark nail polish on finger and toe nails makes them less succeptible to chemo damage (and hides the zebra stripes) and I found I had less nail bed pain when I had polish on.

The easiest way to cover a PICC line for showering is to wrap a couple of layers of clingfilm round your arm, then tape round the ends with wide micropore tape - it's worth doing a couple of 'dry' runs to get the hang of it if you're doing it solo.