The flag of Switzerland is probably the archetypical European flag, and many states in medieval Europe made use of a plain cross on red, as for example Denmark.
In Switzerland it was first used in Schwyz, one of the three cantons which formed the original league back in 1291, and was adopted as the common badge at the battle of Laupen in 1339, as as the flag of the confederation in 1480.
Its use was very restricted until 1848 when it was officially defined and regulated.
Swiss flags for use with flag poles are available here.
All prices are in USD.