Wanted Warhammer 40000 Source Books

My son is getting interested in Warhammer 40000 and I am after some cheap Warhammer 40K source books so he can build an idea about the 40K universe. I am after any old codexes or rule books, anything like that. It doesn't matter if they are out of date or anything he just wants to learn about the universe.
I am also after the specific Index Astartes space marine source books. I have book 2 but need book 1, 3 and 4.
Any help would be much appreciated.
Cheers, Ken