Where can I find online material specifications?

Going directly to the source organization is usually the best way. For example, ASTM specs are sold online at www.astm.org If you are interested in MIL specs, they can be found at http://astimage.daps.dla.mil/ and are free. If you are interested a specification service provider that will provide you with specs from multiple sources at one location, here are two companies to choose from.

http://www.ihserc.com/
http://www.usainfo.com/