I bought my Civic used, and on the right front door i would like to install custom speakers in, but i am missing the speaker support/bracket/whatever u want to call it...where could i purchase/obtain this item?

 

Check a junk yard, ask best buy or any sound shop.

 

I had the same situation, I chose to make speaker rings from MDF that bolted to the door and the speaker bolted to that, much safer and it removes the chance of the speaker trim being bent. i.e. something more solid than the stock plastic junk to mount onto.
Cheers

 

definitely do the MDF thing - it's very easy with a jigsaw or a router