This link is superb.
http://data.ecmwf.int/data/
Click on ERA40 on the left, then data usage, conditions. Read conditions and fill in registration details and then accept.
After that enjoy. It has access to data going back to 1957 with 4 SLP charts per day. I've just viewed every midday SLP chart
for the winter of 62-63 in an animation.

Regards,

Rob
Chester, UK
http://www.west-cheshire.ac.uk/weather/