There are a lot of tubers on the Deerfield River below Zoar Gap. Nice scenery and clean water. Bring your own tube.
I'm pretty sure that they do tubing on the Housatonic - contact Clark Outdoors
There is a company that arranges tubing on the Farmington River through Satan's Kingdom.
Are you looking to tube the Blackstone (not recommended, water is not safe) or SUP the Blackstone. There are lots of access points to the Blackstone along the bikebath from Valley Falls up to Woonsocket.