Check this DIY project man gave you all the info.
I recently changed the woofers in my Martin Logan Monoliths to Focal Audiom. Big improvement!! (I'm using an active cross-over, so I could get away with a change this dramatic). I found these used, so my cash risk was reasonable. If you go to Stereophile and look under links to die for there is a DIY section. You can find places that sell drivers there. I've also heard the Cabasse drivers are very good--but have no first hand experience with them.
You first need to decide on what type of enclosure you will be employing. Sealed box woofers are VERY different from those used in a vented alignment. Other things to consider are what size of driver you are looking for, what type of music you will be listening to, and your price range. Domestic(US) woofers are much cheaper than most European brands. And not always inferior. For a sealed box, I have found success with Audio Concepts(EXTREMELY underrated), Dynaudio, and Eton. Good vented woofers are manufactured by Focal, Madisound, Peerless(underrated), and Swan. Please let us know more about your design, if you want more specific information.
Depends a lot, too, on how much you are able and willing to spend. There's also the other driver(s), crossover and cabinet. Also to consider is whether you have any preferences for size and technology (sealed, ported, transmission line, or horn-loaded enclosures), sensitivity, which influences what amplification is appropriate (the range seems to be from perhaps 86 to 100 dB/w/m), frequency response ("monitor" or "full range" -3 dB @ 20-30). There's also the possibility of using used/vintage components (Altec, Tannoy, JBL, etc.) or using very expensive new components like TAD---although the four previous are all high sensitivity (efficiency) drivers that require ported or horn-loaded enclosurres. . . .Having said that, I have a pair of 15" two-way Altecs in the original sealed enclosures and they manage 94 dB. . .Have a lot of fun, whatever you get and whatever you build and keep us posted! Regards, Dr_joe.
Carbon Fiber, Carbon Fiber, Carbon Fiber!!!! I had a buddy do a comparison test between many different brands of woofers which utilized many material topologies (stuff like kevlar, Polypropolene, etc). He found one thing in common with the carbon fiber varients that he tested: They were much less colored in the upper bass/lower midrange frequency regions, than any of the other materials utilized employed in the the other woofs.(Unless you plan on crossing over your woofer below 100hz, I strongly consider going with a carbon fiber unit). If I recall, the Vifa was deemed the best overall sounding driver.