If the barbs have o-rings on them, you aren't going to find them in your local hardware store. The hardware stores only carry NPT barbs (tapered threads), while barbs with o-rings use straight threads. If it is using straight threads, they are more than likely G 1/4" as that is the most popular thread used, but I can't find anything on their website that lists the barb thread size.
That waterblock is using a very antiquated design in the vein of the old Maze 3 blocks. You really would be better off getting a decent block than messing around with this one.