Vishay's Siliconix brand offers a full line of small-signal and power FETs, including JFETs and MOSFETs. Their series of small-signal application notes is a good source for basic JFET info useful for designing hi-Z buffers and gain stages.
AN-32 briefly presents over three dozen FET circuits. Good candidates for interfacing piezo pickups are the "Ultra-High Z_in Unity Gain Amplifier," the "Hi-Fi Tone Control Circuit, "High Impedance Low Capacitance Wideband Buffer," and the "High Impedance Low Capacitance Amplifier." I have used a modified version of the latter circuit in some of my own custom preamps and it works well.
The links below point to scans of a hardcopy printout of a web page that no longer exists. It was hosted by David Forsyth of New Zealand, and its contents were drawn from an article in New Zealand Musician by Stephen Delft. I have not built the circuit and cannot vouch for it.
This document, archived at Harmony-Central, collects posts from various guitar-related Usenet newsgroups offering advice and schematics for DIY preamp builders.
The link below is a scan of a FAX sent to me by another audio DIYer, Dan Houg, who himself received it by FAX from Jensen Transformers. One could use the front end (up to and including C3, omitting the transformer, D1, C2, R6 and R7) and the optional battery power as an unbalanced FET buffer. Used this way it provides no gain, and is not well-suited to driving a long cable or a low impedance load (the transformer adds those capabilities).
This Jensen schematic details an onboard preamp that is remotely powered. The main purpose of the transformer (which would be located in a remote box powering the onboard unit) is to isolate the "phantom" power from the audio signal. The onboard preamp part shows a topology for a buffer with a low-Z output and with minor modification could be powered by an on-board battery.
Rane provides not only manuals but also schematics and printed circuit board stuffing guides for their gear online. At the AP 13 page you'll find the schematic (300k PDF) for this highly regarded dual-source preamp. The pickup channel uses a BiFET op amp (the Burr-Brown/TI OPA2604) as a high impedance gain stage. Op amps are somewhat problematic in this application (because of nonlinear input capacitance that produces distortion when used with high impedance sources). But Rane has selected an op amp with properties that reduce this effect. In any case, the schematic illustrates how to use a noninverting op amp stage as a hi-Z gain stage with adjustable gain.