Hi, I'm going to sell my DKM signed poster and have been told it's best to get a COA, would anyone have any idea how to go about this and the cost?

Poster is 41/90 by artist Brian Mercer and fully signed by band and artist, a silk screen poster

My wife and I got the autos in person at sound check for their Vicar Street, Dublin album launch gig, they did a meet and greet with all proceeds going to Claddagh fund, each person got a poster.

Have full pics to authenticate