you could make one out of a canister filter but the only problem would be that the flow going through the media has to be very slow, I have a max jet 400 running at aroud 1/3 the flow through mine.
The Two Little fishes phosphate reactore is not that much money and if you have a spare power head kickin around it would be better to just buy a new one.
http://www.jlaquatics.com/phpstore/s..._ID=tf-reactor