I see them frequent on Ebay.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Smith+and+wesson+magna+style+grips&_sacat=0
Originals .. even used can get a bit pricy but mostly around $100 ish for plain old school factory but can go up fast depending on condition and the position of the stars.
Lots of after market places though and depending on the gun you can find them around $50 and under.
Authentic is going to be a fair bit more.