You may not need legume inoculants. Many soils have plenty of varieties of rhizobia which live on from year to year . Pull up some bean plants. If you can see small pinkish white nodules on the roots, the soil has the right type of rhizobium.
If there are no nodules, or in the following circumstances, your soil will benefit from an application of inoculant.
• Where the topsoil has been removed. Exposed subsoils need to be transformed in order to be productive . Rhizobium bacteria are certain to be nonexistent or greatly reduced in such situations.
• When a soil has been fallow for a long time without any legumes in the ground cover. Rhizobia can lie dormant for several years, without an active association with a legume's roots. With time, however, dormant rhizobia die.
• In soils that have been heavily sprayed or fertilized with chemicals. Some synthetic fertilizers are caustic to the bacteria, and copper and mercury sprays are toxic.
• When planting a variety of legumes that have not been previously grown in your garden.
To expand on Mike's comment, just keep your eyes open when driving about town. If you see a particularly healthy plant specimen in an open field or a ditchbank, go on the assumption that it has a healthy symbiosis going on with some soil microbes and yank it up. If you know of a place where there is a particularly vibrant stand of clover, there's your legume inoculant. If you see an oak tree that has a particularly heavy flush of mushrooms after a rain, there's your mycorrhizal spores.
There is also an advantage to collecting locally than relying on mail-order from someplace 2000 miles away; the microbes you pull yourself are already adapted to the local climate and biological competition.
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