If you need a replacement antenna or antennas (Airport wi-fi or bluetooth), search on Ebay for Apple 076-1352 to locate the official Apple parts zip lock clear "baggie" containing one large Airport antenna and two smaller identical bluetooth antennas for the 2009 Mini.
As of this writing, Ebay has two listings selling the three-antenna set for $9 (both are "buy it now prices" with free shipping). There's also a vendor selling it for $69 -- WAY WAY too much. Sometimes you can also find someone selling only a single bluetooth antenna or two or a single Airport antenna.
Keep checking back on Ebay if nobody has this part at a cheap price -- it pops back up from time to time at varying prices, or set up the search so Ebay automatically notifies you when the part number is newly listed.
The usual Apple parts suppliers that I tried some time back did not have it in stock anymore.
I don't think it would be too hard for a do-it-yourself-er to replace any of the antennas by following the iFixit link given by Fischersd above. Just note that the leads on the antennas are easy to break off if you are not careful and patient. (I know this from personal experience in breaking a bluetooth antenna lead I was trying to reinstall during a hard drive swap on my 2009 Mini).
If you are not comfortable with doing the repair yourself, go ahead and obtain the replacement antenna kit to give to a Mac repair shop to do it for you, hopefully at a very reasonable price.
Good luck!!!