DF,

Here's the link to Nasco Farm and Ranch.  I didn't realize they had so many other ventures:

http://www.enasco.com/farmandranch/

The Penn State forage sampler is the "Cadillac" of hay samplers:

http://www.enasco.com/farmandranch/ProductDetail.do?sku=C06541N

You can buy the individual components from Oakfield but the complete kit from Nasco looks to be a better deal:

http://www.soilsamplers.com/prices.html

The Oakfield is also available from Gemplers:

http://www.gemplers.com/a/u/roh.asp?UID=U20060526x3118011x751&GEN2=&GEN3=&GEN9=0

I read somewhere that you can take a golf club shank, sharpen one end, stick the other end in a drill and use it for core samples.  I bet there are some broken clubs in the lake over at Firestone.....   :)

I think your idea will work just as well or better.  The hole saw should be smaller than the pipe to make it easier to remove the core from the pipe.  Use a dowl rod to push the sample into the pipe to the drain hole.

The experts at OSU say to take 20 samples for accurate results.  Pull the core from the center of each bale.