Scamp, and other, use the 7" brakes because that's what the
axle manfs use on that small
axle (Dexter #9) -- You can't substitute 10" brakes because the spindles and hubs are different -- You would have to change the
axle out for a #10, rubbered at 2,300 lbs to use 10" brakes.
There are numerous sources for 7" parts, like Redneck Trailer Supply, Scamp, Trailer Parts Depot, Northern Tools, RV dealers which sell pop-ups, etc.