Apparently, Comcast has not carried Boomerang since 2016. It may or may not have something to do with the fact that Boomerang is owned by Warner Brothers Entertainment, which is an AT&T (read: "the competition") company. I was going to suggest that it might have something in the On Demand section, but I couldn't find anything. You may have to subscribe to something like Hulu to get it.
There's a Boomerang channel on Roku, but I'm pretty sure there's a monthly fee for it ($5/month or $40/year).