I really love the TortoiseSvn diff software. I can download them standalone as a zip, but I really love when iI can select two files and then right click and from context me
You can't, sorry. Yes, you can!
According to this forum thread (from June, 2011, so pretty much over three years old), written by Stefan Küng, an author of TortoiseSVN, starting from version 1.7 of TSVN, TortoiseMerge does not ship as standalone.
You can go to Tools folder on TortoiseSVN's site at SF and get TortoiseDiff-1.6.7.zip file from there. It still quite outdated (see above notice), but certainly it is a standalone version of TortoiseMerge.