usually the stock is behind and the forearm is in front of the receiver.
You can request one from Remington for free at their web site.
Check on ebay. You will find plenty of synthetic options and occasionly some wood sets. Most 7400 forearms and stocks will fit 742 models.
You will not likely find a Remington stock. I put my 722 in a Bell and Carlson stock and I love it.
Try contacting Mossberg or e-gunparts.com
You can find it stamped under the forarm on the under side of the barrel and on the forarm its self
The stock for this model gun can be found at cabella's
Try e-gunparts.com
can find on ebay for around 80.00
The sportsman 78 model Remington is the same as a Remington 700 adl, but i have yet to find whether to buy a short or long action stock for mine... so i would just go for a rem 700 adl stock
The Remington Model 740 semi-automatic rifle has long been out-of-production. You may be able to find an original stock on one of the firearms auction sites, like Auction Arms.com, or you might try ebay. Consider that the later model rifle stocks, like the 742, 7400 and 74 Express stocks will also fit, but will not match the 740 Model forearm pattern.
Try Calbelas Shooting Catalog, that's where I got my stock and forarm for my auto 5 light twelve. Why they named it the light 12 is beyond me.
Check the list of vendors on the Remington Society of America here: http://remingtonsociety.com/rsa/service