Circuit boards
You can make your own printed circuit boards without too
much investment of time or money (of course, you can also
spend more and get "prettier" boards too). A detailed
discussion of the work involved is coming soon to the
BEAM Reference Library's BEAM
Techniques section. You can find PCB "artwork" in a
variety of places; Bill Bigge has some particularly nice
designs at his Board
Room site.
Integrated circuits
Some circuits are just handier when they come on a little
"chip." For things you anticapate swapping in & out
of a circuit (either because they're "smoke-prone," or
because you're swapping things around to compare
performance), you can build your own simple ICs. A
detailed discussion of the work involved is coming soon
to the BEAM Reference Library's BEAM
Techniques section.
Gear motors
You can just as easily buy commercial
gear motors, but it's cheaper (if much more time
consuming) to build
them up from DC
motors and home-brew gearboxes.
Structural pieces (legs, etc.)
There's really no way around building these yourself.
Various assemblies
Various pieces of electronics are buildable yourself.
Motor drivers, sensors,
solar engines -- all are
addressed in this site's assembly
descriptions.