you can pick up an OK cheap (£30 or less) pickup off eBay (J-tone are decent quality for the price) if you can't find one on BC (would help to know your budget) http://www.ebay.co.uk/sch/i.html?_sacat=0&_from=R40&_nkw=double+bass+pickup&LH_PrefLoc=1&_sop=2
Piezo pickups have low output and sound whiny through a non-impedance matched amp which is why most DB'ers have a preamp in their bag (some amps do match, eg, Genz, Acoustic Image, Euphonic Audio). The cheapest one is the Fishman basic preamp but it is no frills compared to the more expensive Plat Pro which has phase control, boom control, more sophisticated EQ etc http://www.amazon.co.uk/Fishman-B-11-Acoustic-Instrument-Preamp/dp/B0002D0JRY/ref=sr_1_11?ie=UTF8&qid=1402932801&sr=8-11&keywords=fishman+bass
You can do simple impedance matching by putting a Boss tuner in your signal chain as a fallback option
The main issues with amplifying a DB are (1) feedback (2) boominess and (3) reproducing a DB-like tone. Its a never-ending and expensive search unless you get lucky. A decent piezo pickup and preamp will sort you in most circumstances