Data's arm from the episode "Phantasms" is up for sale at Prop Store. It is rather expensive, but cool none the less.
From Prop Store:
Data’s (Brent Spiner) hand and sleeve from the sci-fi television series Star Trek: The Next Generation. The piece was made for the sixth episode of the seventh series, titled “Phantasms,” for a nightmare sequence experienced by Data where he was attacked and dismantled by workmen.
The hand has been cast in latex and painted gold to match Data’s skin tone. The hand extends up into a forearm, also made from latex which is covered by a length of a uniform sleeve with a brown satin lining. A thin metal rod is located within the arm which was held by the actor during filming. There are slight chips and marks to the paint on the hand. The piece remains overall in good condition. Dimensions: 53.5 cm x 17 cm x 7 cm (21” x 6 ¾” x 2 ¾”)
Item size - 36.61" x 6.69" x 2.76" (93.0cm x 17.0cm x 7.0cm)
While an interesting item, you would have to be a total Data nut to shell out $ 5,989 for this prop. That is an rather high price for what I would guess is a $ 3,000 prop (and that again would be for someone who just loved the character).
Also, what is the provenance of the item? Where did it come from? I am always big on this. Not that it is likely that this is a fake. None the less, every prop seller owes the buyer the provenance of an item or how it was authenticated.