Building your own Sanjiegun.
A traditional, super-basic Three Section Staff can be easily made out of wood, with rope joints where steel is not possible. The ideal Sanjiegun would be instead carbon fiber, with aluminium joints with ball-bearings: this configuration would be the top-choice, but everyone knows that martial arts weapons are such because they can be easily made when battle is near, so a half-way option is still the best in terms of availability of material, time for building the weapon, and cost.
Note about carbon fiber Three Section Staff: undoubtedly the best idea for super-fast action, but the worst when comes to impact, as sticks might slowly wear out at their edges on each impact.
It really depends on you, your height, and the length of your arms, but as a a general rule, dimensions are: 55 cm for the middle-section stick, 61 cm for the outer ones.
Ball bearings provide a smoother movement, although you should consider that the Sanjiegun was first built using rudimentary joints, without causing any problem in battle. Ball bearings were introduced just lately for the practitioner to learn with total absence of "bumpy" movements.