Your bike is English threaded with a 68 mm wide bottom bracket shell. That is a VERY common type and by far the most available configuration so finding replacement parts will be easy. Remember, the drive side cup of the bottom bracket is LEFT-HAND threaded when you go to remove the old one or install a new one.

Your current 105 triple Octalink has a 118.5 mm spindle. A double Octalink crank will require a 109.5 mm spindle. Since road Octalinks only came in those two lengths, the choice is pretty easy. 105 Octalinks (BB-5500) are still readily available in both lengths. You should be able to find compatible NOS 9-speed era 105 (FC-5500) or Ultegra (FC-6500) double cranks if you want to keep the Octalink type.

Your bike will also take any of the newer external bearing bottom brackets and cranks such as the current Hollowtech II 105 (FC5600) or Ultegra (FC-6600 or FC-6700) in either double or triple form. Since the spindle is built into these crank arms, there is no choice of length to make. These are "10-speed" cranks but will work ok with a 9-speed chain and cassette.