Q: iMac 2007 change hard drive

Apple considers that model "vintage" and will probably not work on it. There may be exceptions as shown in this Apple support article:

Vintage and obsolete products - Apple Support

It is a potential "do it yourself" task but does require dealing with teh fiddly bits. Select one of teh hard drive installation videos on tis page:

http://eshop.macsales.com/installvideos/imac_mid_2007/

and you can assess whether it is something you wish to try at home. Note: specialty tools are probably needed.

You are likely to find an Apple Authorized Service Provider (AASP) in your area that is willing to do the work. Instead of putting in another HD, you may want to consider replacing the HD with a SSD, this will speed up performance considerable. You can begin finding a AASP by clicking https://locate.apple.com/country