Shoot I got tons of those, they are common. Well maybe not tons but I have a lot of cords. Check thrift stores, Habitat Restore, etc. If you can't find one PM me.
Yes you can shave off some of the squared side of that cord to fit it in the boombox, no problem. You can also find a replacement cord in the hardware section of HomeDepot or ACE.
First I'd go digging in the bins at Goodwill etc.