Downloading old versions?

http://static.wordpress.org/archive/

But you don’t need to upgrade sequentially. Each download is a full install, so if using 1.5, just download and replace your current installation with 1.5.1.3, then run wp-admin/upgrade.php.

Unless you’re trying to update only the files that have changed, the instructions are the same.

Follow the 1.5.1.2 to 1.5.1.3 instructions (you can leave wp-contents/ as is):

http://codex.wordpress.org/Upgrading_WordPress#Upgrade_1.5.1.2_to_1.5.1.3

But do make sure to run upgrade.php at the end, as there were a few changes since 1.5 to the options table that need to be added.