Hello Climber,
We all find it hard to get those parts.
A and B have the same answer, you can get some frame parts in shops that sell frames for pictures, but not in all of those. Some hardware stores do also stock parts and model builder shops might be able to help you, either by selling or by sending youon to someone near.
An other way is to buy secondhand frames, carbootsales, garage sales, charity shops and so on.
C. For the brass plagues, I never heard of someone who bought them ready made in a shop, but some shops will make them to order, expect to pay fortunes.
If you can get the cheap, badly made, knotboards with the right kind of plaques, buy them and re-use.
Most of us will just do without. (In English you have a chance to buy them, in Dutch it is even harder, and some of use speak languages with even less speakers.)
D. You did not mention which part of the world you call home, but if you are in England, try Footrope Knots, Des Pawson for cotton cord. knots@footrope.fsnet.co.uk
501 Wherstead Road, Ipswich, IP2 8LL
In America there are some others, I do not have their addresses but some other members here do.
I hope this is some help for you, I know it is not much but it is a well known problem with few answers.
Willeke
Sorry, I missed the heading of the topic, UK....