How prevalent is sexual dimorphism?
I would say that anisogamy, the different gametes such as sperm and eggs, is itself, a form of sexual dimorphism. However, you've since stipulated that you want to know the prevalence of sexual dimorphism beyond sex organs/gametes.

Sexual dimorphism occurs in a vast plethora of traits, at a variety of levels, and in almost all known dioecious species. It occurs even at the level of gene expression in, for example, mice, birds, mosquitoes, fruit flies, nematode worms, humans, and much more. Sexual dimorphism is prevalent and obvious in morphology such as peafowl, red deer, mallards, humans, guppies, fruit flies, horned flour beetles, and much more. It also occurs in physiology, behaviour, and life history traits (e.g. lifespan, ageing).

Simply put, sexual dimorphism is extremely common. It is hard to put a number on it, any studies of sexual dimorphism are likely to be focussed on sexually dimorphic traits (so a meta analysis will likely suffer from publication bias), and not all traits can be easily measured. However, gene expression is a good place to look because many traits (expression level of each gene is a trait) can be sampled simultaneously. This study found that 18% of genes were significantly sex-biased in any one tissue, while this suggests that at various developmental stages it was between approximately (based on table 1) 13% and 61%. Note again, there's not much literature which actually puts numbers on the general frequency and extent of sexual dimorphism. That is generally because, in the relevant research community, the gonads and gametes are included as sexual dimorphisms.*

Also, despite the frequency of sexual dimorphism, it is thought that the evolution of sexual dimorphism is severely constrained by the sexes working from the same genome (links: 1, 2, 3, 4, 5, 6, 7) and sexually antagonistic selection could readily favour further evolution of sexual dimorphism.

In terms of your report, I don't think you would need firm numbers or a citation; it's a well accepted fact that sexual dimorphism is common.* You would be more than fine to say something along the lines of:


  "Sexual dimorphism is extremely prevalent in nature, found in a
  range of taxa, and found in a variety of traits."




* These points are based on my experience, I did my phd thesis on the evolution of sexual dimorphism so I've read quite a substantial component of the relevant literature.