Hi, as Rosemary says, the SCF are a good start. They have recently started publishing details of available Crofts.
The other option of course is estate agents and solicitors in the crofting counties, some agents tend to deal in the sale of Crofts more than others but if you choose an area and keep an eye on the agents you will see them.
I'm assuming you know all about the constraints of crofting and the fact that buying a tenancy so you rent it is pretty much the same cost as becoming an owner occupier, hence so many young folk can't afford Crofts.
Croft land has significantly increased in price in some areas over the past couple of years. I've recently seen Crofts on Skye sell for in excess of £100k and that's without a house or buildings on it.