can i get a cd changer which will attach to my existing radio/tape player for my 2006 old shape punto or do i need to replace the head unit?

You can in some cases but not sure about the Punto unit. If they aren't avaliable from Fiat then you won't get an adaptor for another make such as Sony. If you can get a CD Changer from Fiat, then in some cases Sony and Kenwood can supply adaptors to make their CD Changers fit existing head units. You're best bet is to pop down to your local Halfords/Audio Store and ask them to find out for you.

Second one down here MIGHT work, http://www.caraudiodiscount.com/acatalog/fiat_cd_changers.html

I can't guarantee it though, do a google search for 'fiat punto changer interface'

Connects 2 do an adaptor to a Sony changer, if your head unit is a Grundig
AD182H/H2/H3/AD185H/H2 then these guys do the cd changer and adaptor for £139 and offer a next day delivery service.