There are hundreds.
What application? How much power do you have? What frequency range will they be playing? What type of install (IB in door, small sealed, ported?)? Passive or active crossover? What tweeters and midrange (if any) will you be using?
That's just to start with.
http://www.partsexpress.com
http://www.solen.ca
http://www.speakercity.com
http://www.meniscusaudio.com
http://www.zalytron.com
http://www.madisound.com
http://www.diymobileaudio.com
Are just some places to start looking.
What passive crossover?passive crossovers,
x50 thousand.What passive crossover?
You are going to need to get the same impedance that the crossover was designed for (if it was designed for 4ohm, you'll need a 4ohm mid). Likewise, you are still going to need to know the crossover point and slope....no point in recommending a mid that's only good up to 2khz if the crossover point is 3.5khz @ 6db/oct.....it'll sound like poop.
Also, does the crossover filter have any built in fancy things like a zobel network? That will affect what speakers will and won't work aswell.
Or are the passives going to be custom built?
Lots of things to consider here.
What crossovers? //content.invisioncic.com/y282845/emoticons/crazy.gif.c13912c32de98515d3142759a824dae7.gifx50 thousand.
what crossovers?