Download available at: http://sourceforge.net/projects/realboy/
If you use the ‘pkgsrc’ portable packaging system (for example, the NetBSD Operating System), you can find a suitable package at: http://pkgsrc.se/emulators/realboy
Important work has been done by Joao Fernando Coelho Lopes <email@example.com> and Fernando César <firstname.lastname@example.org>, including support for the MBC2 memory controller, a profiler to check for the most-used instructions so as to improve instruction emulation through predecoding (which he also implemented). The code hasn’t been merged yet to the main branch; you can find his work here.
RealBoy has also been conveniently packed in operating-specific packages, including SmartOS and OmniOS.