You can train with one of the four volunteer organisations, NCT, LLL, ABM, BfN - all have websites and helplines where you could ask for more information. They're all mentioned several times on this board, so I expect a search would yeild something.
The training covers similar ground in all four, though they do have some organisational differences - for instance, NCT covers all parenthood issues, and the other 3 are solely bf orgs.
They are all volunteers most of the time. Time to train is about 2 years.
You can also train at a much more mother-to-mother level as a peer supporter. There are a number of schemes for this - your HV or midwife might know what goes on in your patch. A peer supporter is usually trained in a couple of months or so. She is also a volunteer.
Sometimes, bfcs are paid by the health service as they sometimes attend clinics or drop ins.
Your story is a sad one - I don't suppose your cousin in law could be prevailed on to write and tell the hospital how poorly they supported her bf? 18 bottles in 4 days?! Refusing her permission to see someone?
I agree with the ridiculousness of promoting breastfeeding and then not backing it up with support