According to this Technical note: TN2110, J2SE 5.0 (which is synonymous with JDK 1.5) is only available with releases for 10.4 and 10.5 (see table 3 about three-quarters of the way down the document). On Apple's Developer site, you can still download Java packages from the past (note: you will need a developer or iTunes or apple account to see this page).
I have not tried to install an older Java install from Apple, so I am unsure if any of these installs will give you a JDK 1.5 that is "unlinked" from JDK 1.6 or if these old installs will place nice together or just replace. In your Utilities folder there is an application called
Java Preferences. If you launch that, you can see all the Java runtimes that you have installed already. Before you try downloading past Java installs, perhaps you can use that tool to see if you have a more complete JDK 1.5 installed.
Sorry I cannot give you a more concrete answers, but I wanted to at least give you some leads to try.
Also, have you tried checking the various support forums available for Netbeans? Perhaps another Netbean Mac OS X Snow Leopard user has already found and published the answer...