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If you can't get a hold of 6.1, you could try going to the Version Conversion board and asking very nicely for someone to convert it to the version of LabVIEW you're running.
LabVIEW prior to 8.0 had no real license manager integrated so you could never download anything but a specially compiled evaluation version for it. If you have a valid license for a more recent version your local sales rep may be able to help you with an installation, but it's actually also quite possible that they don't have that old versions laying around anywhere. After all collecting everything only takes up space and does collect dust.