that's a very nice machine!
And for fun - Singer converted from the 27 to the 127 around 1913, so you've got a very early 127 there.
SewClassic is a very good site for supplies- belts and bobbins and such. Treadle belts are readily available and affordable. You get one on the long side, cut it to fit, punch a hole in for the staple and you are good to go. Word of warning -with Singers, you need to "derail" the belt before you put the machine away, since the machine needs to tilt backwards to flip up the front support.