I know they are not easy to find lately and this is the best deal I found.
http://www.shopremingtoncountry.com/Gauge-ShurShot-Assembly-Obsession-F401363/dp/B00JF68UDE?field_availability=-1&field_browse=3414329011&id=Gauge+ShurShot+Assembly+Obsession+F401363&ie=UTF8&refinementHistory=subjectbin%2Cprice%2Csize_name%2Cgeneric_text_4-bin%2Cgeneric_text_3-bin%2Cgeneric_text_10-bin%2Ccolor_map%2Cgeneric_text_2-bin%2Cgeneric_number_1%2Cgeneric_number_2&searchNodeID=3414329011&searchPage=1&searchRank=salesrank&searchSize=12
http://www.midwayusa.com/product/145093/remington-shurshot-stock-and-forend-remington-870-20-gauge-synthetic-black
Here's a link for 20 gauge stocks