Consider that a huge variety of 90 and 105mm macro
lenses have been made in all number of different camera
mounts.  The 105mm in particular is a "sweet spot" for
optical design, and virtually every one of them ever
sold was a relatively "good" lense!  The older lenses
made for mounts such as the Nikon, that can be used on
modern cameras, are becoming a cult item and the prices
are going up; but those made for mounts that have been
abandoned are often very inexpensive.

Another source for excellent macro lenses are the now
dime a dozen 50 to 150mm enlarging lenses.  Not all of
them are good quality, so knowing which is which makes a
difference.  El Nikkor, Wollensak, most Rodenstock, and
others are great lenses.

Another lense option would be achromatic microscope
objectives.  These generally will be better for greater
than 1:1 magnification, and probably will work best with
smaller than "full frame" sized sensors (due to lack of
coverage for larger sizes).

The trick is to mount these lenses to a modern camera is
some useful way.  The only part of the cited URL worth
remembering is the use of modified caps for the camera
body.  Otherwise, T-mount adapters of various kinds,
step-up and step-down and reverse adapters are all
useful and inexpensive.

For macro work it matters not if infinity focus ability
is lost, and hence using extension tubes or a bellows
can be useful to get the desired magnification.  And
since various adapters are required in any case there is
no need to stick with a bellows that matches the camera
mount.  It's a matter of which adapters can be found for
both ends of the bellows!  An old bellows with T-mounts
on both ends (Vivitar, for example) is probably the most
likely to be useful, but Pentax screw mount and Nikon
bayonet mount are also useful because of the large
variety of adapters that are available.

Finding adapters of course can be an art too!  EBay is
one source.  I've bought a lot of that type of thing
from fotodiox.com and have always been pleased.