Which version of the Titec bars do you have? - the 118s are incredibly thin walled and would definitely need support, but the regular ones should be fine without, if you're not over zealous with the allen key.
If you do need the plugs, I wouldn't be too concerned about the spec's. As long as they're a reasonably close tolerance fit and don't rattle about (slight deformation of the bar when clamping will hold them in place), 20 - 30mm long and a mm or two thick they'll do the job.
The 118 are slightly prone to bending in a crash, but the ends are quite hard to permanently deform. They’re fantastic bars!