Are there such things as after market suppressors for springers
6MM,
Not sure about springers, but take a look at TKO's website. They make airgun suppressors.
I've got one for my Benjamin Discovery PCP Rifle, very effective. Almost as loud as a standard velocity .22 rifle without the suppressor.
And of course since it's not a firearm there's none of the NFA paperwork, tax or wait.
Jerry