Hi Dans
We have in the past got horns back from the abattoir, but only large ones from tups. I say 'got back' but in fact Mr F had to go to the abattoir and saw them off the amputated heads himself. Not a job for the squeamish.
The inside of a sawn off horn is partly hollow, to allow for the living core, which you can feel as warm if you hold a sheep's horns. This means that if you are wanting to make something such as a crook, then the useful length is much less than the total length of the horn as seen from the outside. Crook makers really want horns from mature tups of maybe 5-6yo, and even then pay only about £10 per horn.
Ewe's horns are much smaller, so not really very useful.
When you get your horns back, there are various ways to treat them. We put them on the roof of a shed as far from the house as possible, and leave them there for a year for the maggots and the elements to do their work. The previously living core of the horn has to be degraded to something which will fall out when it's shaken. I'm wondering if perhaps there is a way to remove the horn from the head, in such a way that the core is left behind, so cleaning would be quicker. I can think of a possible way to do this.
Alternatively you can bury the horns for a year, but do mark where you put them - we still haven't found the roebuck skull we buried years ago.
A third option for bones is to boil them in, I think, something like washing powder, but I'm not sure that would work with horn which is made of keratin.
I haven't thought of needing to sterilise the horn, as a year out in all weathers must surely clean it enough.
Working the horns yourself is not difficult, basically it involves slicing and polishing. The hollow part of the horn, sliced, makes a nice pendant, or earrings, and the tips can be turned into toggles. A small vice and tiny drill set help but are not necessary for your initial experiments. Making a crook is a skilled process, and involves shaping the horn using heat initially, then carving the shape, and polishing to a high shine.
Good luck
FW