Don't overlook lens hoods from manufacturers other than Nikon.
Also consider padding out the HN20 hood with a spacer. If you have a few old 72mm filters with ruined glass, you can make your own spacers - just remove the glass from the filter and pad out the lens with the glassless filter ring(s) before adding the HN20 hood to get extra shading.
In respect of hoods from the other manufacturers, see what is around in the 72mm screw sizes for both 135mm and 150mm focal length lenses if you want a better shading hood for the 135mm DC lens. A hood for a 150mm lens can always be slightly reduced in length with a Dremel if vignetting is encountered. In the case of Nikon hoods, they are pretty conservative, so quite often the hood for the next longest focal length usually will work OK on a lens with the next shortest focal length if not too many filters are between it and the lens. A step up ring can also help use a hood from another lens if you are not stepping up too much.
I buy up lens hoods from various makers at photographic flea markets for this reason. I am brand agnostic with respect to lens hoods - so long as I can screw or clamp them onto my lenses.
I probably should list the HN-20 that Bjørn mentioned. This hood is for the AIS 85/1.4. The overall depth is greater than the built-in hood for all 135/2 versions, and it is more solid, but it also flares out much more so does not offer optimal shading. Among the Nikon hoods this is the only practical alternative - other Nikkors with 72mm filter size are long lenses with built-in hoods or wide-angle lenses where the hood is far too short. There may be other third party screw-in hoods which are suitable.
The built-in hood for the 135DC is better than nothing, but on the short side. The hood for the 105DC is similar but the front element is deeply recessed so overall that lens is better shaded than the 135DC. Both hoods screw firmly in place when extended, not like the wobbling slide-out affairs of the AIS versions. Note the threads are very finely pitched and prone to cross threading so take care not to jam it.