Joe,
I don't sell stuff. Try
www.helenhowes-sewingmachines.co.uk for feet and attachments.
3/6 (or 3s 6d) is 3 shillings and sixpence, old money, replaced by the decimal system in 1971. There were 12 pennies in a shilling and 20 shillings in a pound. So in new money 3/6 is 17 and a half pence. I'll let you convert it and allow for inflation.
Stoppaxes are very hard to come by if you want one. Helen Howes has one for £25 because it is so rare. I found one in the odds and ends with a sewing machine I bought a couple of years ago and searched the internet and found the instructions. I haven't tried it out yet - I've only just started FMQ and have used a hopping foot, like the one at the bottom left in the second picture.
A video on FMQ is on the list of things to do, but I will have to improve first!