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If you don't want to or can't install a Sustainiac pickup, you can get a much cheaper handheld one-string "E-Bow" that does the same thing. It's not as easy to use as a Sustainiac and you can't also be playing with the whammy bar unlike with a Sustainiac, but you can get it to do tricks a Sustainiac can't do: see the "spiccato" section in https://www.youtube.com/watch?v=b0V3pzxma-8

I've also managed to make an E-Bow work with a steel-string acoustic guitar (but only on one string IIRC).