Sako dropped the Model 78 in 1986 which means that by...say...1996 there were no new parts being made..likely sooner than thatand any factory parts soon dried up. Have you gone to the Sako website use www.sako.fi and NOT www.berettausa.com as the beretta site will not contain anything prior to their deal with Sako and see if you can access a list of authorized dealers-service centers for the model. One of those may have some shelf stock. One other source might be the aftermarket magazine maker Krasne's D/B/A TripleK...www.triplek.com other than that check the postings on this website by Alan in Nevada, Alan post a long list of used parts dealers..While at Sako Finland, check to see if there is/was a european version of yoyur rifler that stayed in or is still in production...
Brian try giving Hoosier Gun Works a call. They show Sako 22 Hornet Magazine. http://www.hoosiergunworks.com/catalog/magazine_id_rifle.html
I have a new one I will sell