Courses to take for Ph.D. in accounting

The classes I'd recommend are probably the same as the classes for other quant-intensive areas of business (finance, operations research) -- econometrics, microeconomics. For tax-related stuff, actual tax and accounting courses may help you understand the topics better but likely won't directly help you "perform better" in a Ph.D. program, whatever "perform better" means. I have no clue what tax researchers actually do but I'd imagine game theory/mechanism design courses can be important if you want to see the bigger picture and interact with Economics faculty members.

You're right that professors specialized in tax are hard to find. I'd look at state schools more closely than the usual "top schools" (Wharton et al.). I vaguely remember that places like UNC, UIUC (Illinois) and UT-Austin had a few tax people. I think you'd rather go to a place where there is only one good tax researcher than a place with 3 mediocre ones.