No need to buy, free advice.
Media Selection.
This is a very touchy subject but very important.
ALL media should be considered contaminated and hazardous.
All new, pre-packaged, used, or recycled media should be considered harmful. Use ALL safety and personal protective gear.
In the United States silica sand media can be used under OSHA safety regulations using approved safety equipment.
Any new, recycled, and pre-packaged media should  be sifted prior to use.
Use a commercially available or an improvised sifter should be used. A window screen steel mesh will work just fine.
Media is rated by hardness using the MOHS scale. The scale is displayed from 1 to 10 being the hardest.
Here's an example.
1 - equals Talc.
10 - equals diamond.
Most of our customers are those who do metal preparing, rust removal, and paint removal. Let's concentrate on this.
Here's to the most popular media readily available. Other brands are available. Big store suppliers such as Northern Tool and Tractor Supply are offer better deals on media and carry adequate supplies for it's customers.
Coal slang.
Crushed glass.
Garnet.
Aluminum oxide.
Media selection.
Grit size.
Let's keep it simple.
Modern marketing is great for the consumer, media is conveniently packaged, and sold to the masses. 
Popular media is sold in:
Fine, medium, and coarse.
Here's a fact. 1/8 nozzle orifice will  shoot fine, medium, coarse. 3/32 stick with fine.
An easy rule : If you can sift it through a wire mesh sifter (pictured) or wire mesh window screen it will shoot through a stock small pot nozzle.
Shape:
Once again keeping it simple.
See photo media shape.
Most of our customers are blasting off rust, paint, and scale.
Use angular or sub angular.  I added clean sand MOHS around six Crushed glass around six and a half and Black Beauty around seven.
Sub angular is sharp and provides a good profile for most surface preparation.
Rounded media would be the least expensive and can be used for general cleaning, and surface preparation. See chart.
Full blown Angular media is expensive it's usually used in a cabinet due to its excellent cutting abilities along with hardness. Hard media can be recycled several times over, it all depends on PSI/Velocity the higher PSI/Velocity the less longevity. Defragmentation, this is when the media shatters and becomes less effective in removing a coating.  After a while your media becomes a fine powder.