Which virus capsids consist of only one type of capsid protein?
Interestingly, the fact that viruses might be made up of repeating subunits was a hypothesis by the famous Watson and Crick in 1956.

A useful resource for this is the Viral Zone website maintained at by the ExPASy bioinformatics system. It contains an easily browsable and human interpretable list of viral families and their characteristics. In particular, you can "browse by virion" which will show you different structural classifications of viruses; make sure to look at all the different genetic classes of viruses (single-strand RNA, double-strand RNA, DNA, etc). 

From my preliminary investigation, it appears that many capsids are composed of only a single subunit; in addition to TMV, some other examples I found are the totiviridae family of viruses which infect single-celled eukaryotes such as Giardia lamblia, the human-infecting astroviridae and calciviridae families, and (as far as I can tell) the Heptatitis B virus. It also appears that Dengue virus has a single capsid protein.

As for the tertiary structure of capsid subunits, you might find this review of interest. Basically, due to symmetries in the shape of icosahedra (or helices or other geometric shapes adopted by capsids), you can argue that the structure of subunits in certain symmetrical parts of the capsid has to be the same (because they are copies of the same protein and, by positional symmetry, they have to be experiencing the same forces). When there are asymmetries in the structure of the icosahedron, then it is often hypothesized that there exists a state of "quasi-equivalence;" that is, the deviation from symmetry is small enough that the tertiary structure of each subunit is not affected very much. The review goes into these points in much greater detail.