You should search at StewMac (aka Stewart-MacDonald)
http://www.stewmac.com/shop/Nuts,_saddles/String_nuts.htmlAlso look at this site for hints, tips and tricks for installing a nut. Also lots of tools...like files (not cheap). Understand that you might find lots of info and ideas here, but StewMac has a $30 mininum order.
Also look at these sites:
http://www.bryankimsey.com/nuts/http://frettech.com/nutz/index.htmlNuts aren't generally pre-slotted for the strings because every guitar is different and they don't know what strings are being used. The string slotting is done with a set of fine files.
The problem is that without a set of files, you're hard pressed to do a halfway decent job even after you determine the correct nut blank.
This means ending up at a guitar repair place for $60+/- to have it done even after you fooled around with it.
There ARE some pre-slotted nuts that you'd find by simply Googling "pre-slotted gutar nuts"...but getting the correct size is the issue . Just measure the nut space with a small rule to get a rough idea what you're looking for.
You sound like you're about to put more money, time & trouble into a guitar than it's worth. We're living in to golden age of cheap guitars for beginners...and most would probably be a better bet.
Probably not worth the problem UNLESS this is a cool old Silvertone guitar in the style of the early DanElectro models. And even then you should know that those used a ALUMINUM METAL NUT.
BUT, if you're real handy, you could always cut a nut blank out of bone (beef works) and mess around for laughs. Do you know a butcher? Have him get it generally in shape some blanks for you and whittle them down with files and sandpaper. That's what I'd do if stuck on a desert island....with a cow I didn't like.
Seriously, my luthier carves his own nuts often from ostridge bone because it's so dense. He picked up some ostridge bone at a NAMM Show in LA from a guy that lives in Australia and had to fly home with all these bones. He said the Homeland Security luggage inspection was sort of strange trying to explain all the dried bones in his baggage.
In answer to another of your questions, the nut needs to be glued in so it won't move side to side.