Would you consider a mutual fund? If so, then perhaps the PIMCO Commodity Real Return Strategy fund would meet your requirements. http://www.pimco.com/ps/bonds_prodserv_frms.asp?cont=/ps/bonds_prodserv_commodityreal.htm No load shares are available.
Good move to the grains. Those contracts have great liquidity and provide lots of opportunity where margins do not approach that of the financial contracts. Having said that, please do be wary of trading these instruments based on much of the Cycle work flying around out there. I have been trading grains for years and cannot cite a single vendor in the pubic domain who does quality work. my sence is that it is an intellectualy appealing idea for many first traders entering into those markets. The best information for new traders can be found on the CBOT web site, and it is free!
The best information for new traders can be found, right here "Elite trader for new Traders," and it is free!