Hi,
There are some exchange-traded bonds, and preferred stocks that are essentially bonds (trust preferreds). If you're patient and detail-oriented (sounds like you are), you could do your own research and buy some of these individual issues. These are almost all very long-term in maturity, however, and you likely do not want all of your portfolio in long-term notes. By the way, quantumonline.com is a good resource for researching these bonds and preferreds. I'm not really recommending this route, but just pointing out the possibility.
The simplest and cheapest way to own bonds, except for Treasuries, is through Vanguard funds. VFIIX (GNMA), VFICX (int-term corporate), and VWLTX (long-term muni) are three that I would look at. The "long-term" muni fund is not such a long-duration fund as one would expect from the name; Vanguard also has other muni funds with shorter maturities, of course. The muni yield curve is usually fairly steep, however, and it tends to be more worthwhile to go longer in maturity with municipal bonds.
Regards,
Duane