You've been searching for power pistons for quite a long time, for several months I remember however I would suggest you try to obtain pure for your items and in doing so, will hopefully be successful in leading you to the next Tortodrone event, which means more power pistons in the market / available for sale. Buying them now means you will be searching for them in all the wrong places or just wanting to buy them at their highest value.

tl:dr Sell your stuff for pure. Make money whilst waiting for next Tortodrone event. Buy then.

Or...

Much quicker way: Obtain $20 / £20 / 20 euros and buy energy with money then pay Aandy for his power pistons.