While you are at festivals and events you will want to buy beverages from the vendors there, usually served in a single use plastic cup.

The best way to avoid this is to bring your own by anticipating the need. I recommend glasses made of stainless steel but plastic alternatives are also available.

 

UK Made Stainless Steel

Pint Cups: Single or a set of four. [here]

Half Pint Cups: Single or a set of four. [here]

Cheaper options are available on ebay and Amazon but will likely be made abroad.

 

Other Options

Reusable Plastic Cups: If you prefer this option then there are some sold by Drink Stuff that look reasonably priced and robust. [more]

United States: For those in the US check out Klean Kanteen who have 10oz and 16oz stainless steel cups available in singles or packs of four. [more]