where should i buy photoshop and which one?

Well if there was only one Photoshop you should buy with the given information in your title, there would only be one Photoshop to choose from.
 
There is PS and PS extended. The Adobe site goes over the differences. The standard PS complete retail version (not an upgrade) is over $600. You can also check on fleabay and see if someone is selling an older version if you just want it for personal use.
 
If budget is an issue, I'd recommend Adobe Photoshop Lightroom and a copy of Photoshop Elements. I feel very strongly that Lightroom is a much better workflow than just PS alone.
 
^^ I completely agree with Doug. I bought the package of Lightroom and CS3 Extended last year from Academic Superstore and it was very reasonable, it was either $299 or $399.
 
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