What glass and materials are needed to install a glass shower screen against a sloped wall and where can I get them?
Glass in a shower enclosure must be tempered to be safe. Tempered (or toughened) glass is much harder to break than conventional glass and, if broken, crumbles into pieces that are not very sharp.

Because it is so hard, tempered glass is cut to size before it is tempered (hardened). This is almost always done by a specialty glass shop.  Usually they will either take your measurements or come and measure themselves, and then create the sized tempered piece.

There are also various plastics that are used in shower enclosures that are also less prone to breaking than ordinary glass. Some of these may be able to be cut by the DIYer.

In either case, you need to be sure there is a good, waterproof seal between the panel and the wall and tub.