The first thing you need is a CAD model of the part you want to make. After you have the model you can talk to a sand casting house. Depending on the number of parts, you can either have the parts run "loose", or have a match plate made. Running loose limits you to about 10 parts. Match plate will give you hundreds of parts.

Review these pages:

http://www.foundry.ray-vin.com/matchplate/matchplate.htm

http://thelibraryofmanufacturing.com/metalcasting_sand.html

I could be more helpful if I knew what part you want to make and the alloy.

I deal with low volume sand castings daily and I typically 3D print the master and supply to the casting house, or print the splits to be made into the match plate. We use 356-T6