Sorry I don't know where you can buy a bolt secondhand . I have been trying to buy an extra bolt for my 308 Sako for a switch barrel job but no luck.
You may not have to re headspace the barrel. Sakos are made to close tolerances and if you can find the same model bolt there is a good chance it will fit just the same as the old bolt.
I have a few Sakos and one time I went to the range spent the whole day shooting my 308 Sako A11 with my Sako A11 22-250 bolt in the gun. did not even affect the accuracy that I noticed. Did not even realise it until I got home and was cleaning the gun . Kind of embarrassing to admit that but it sure showed how well they are made.
Also I bought a new factory barrel for one Sako , screwed the old one out and the new one in , no headspacing required except a slight body die bump of the fired cases . I would not say that this will happen every time but as long as you check the headspace fit before yo fire anything in most cases the barrels just fit up .
I can hear the gunsmiths gnashing their teeth and kicking and screaming.
These people seem to have spares http://www.midwestgunworks.com/mm5/merchant.mvc
An A11 bolt should fit an L579 action http://www.gunstar.co.uk/Rifle/Sako-...e-gs45671.aspx http://www.gunstar.co.uk/Rifle/Sako-...e-gs41057.aspx
but you would have to headspace it I would think .