There are two classes of materials that contract when a voltage is put across them: reverse piezoelectric materials and electrostrictive materials.

Reverse piezoelectric materials will contract -- though only by about 0.1% -- when a voltage is put across them.  Barium titanate, lead titanate, and lead zirconate titanate are examples of such materials.  If you wanted to get hold of a sample of such a material, a piezo-electric loudspeaker or a piezo-electric cigarette lighter might be the best place to find one.  Barium titanate is also used in some high-capacitance capacitors.  It can be purchased via the Web at 41 for 250 grams, though the price drops to 21/kg if you're going to buy a ton or more.

Electrostriction is a similar effect, found in all non-conducting solids but very slight in most of them.  A few substances -- lead magnesium niobate, lead magnesium niobate-lead titanate and lead lanthanum zirconate titanate -- exhibit it to a higher degree, but still only about 0.1%. I have found lead magnesium niobate available on the Web at 282 for 25 grams, so it's a lot less accessible than barium titanate.