Definitely the Walmart camp gear section to find the cheap 1.5" or 2" quick release straps. A few bucks from Home Depot/Lowes/hardware store for the metal quick releases to connect to the D-rings. Instant harness for back pack trekking with your tube. Fisher nailed it for grabbing cheap alternative, rather than $25 to $30 backpack straps from any company. Plenty of materials to make your straps.
For comfort, you can use pool noodles as cushion.