I recommend ordering tires from Amazon.  I have ordered trailer tires from them twice.  The price was great, delivery was quick (2-3 days), and the tires were recently made (code on the tires indicated they were 2 months old).  I contacted all the name-brand stores in my city and found that choices were limited, some prices were outrageous, and many did not have the tires on-hand.  I ordered Carlisle tires, which are reputable.  I ordered ST175/80R13, D load tires.  I ordered D load because I have a Deluxe 16-ft Scamp, which is heavier than Standard Scamps, and I travel on Interstates back to Minnesota.  A C load tire should be fine for a 13-ft Scamp.  Radial tires give a softer ride and supposedly better gas mileage and longevity.