Check with Tim Clark At "Radiant Heart". He's been selling prayer flags and practicing for a very long time. I only buy from him and while his flags will eventually wear out like anything else, they last a really long time. http://www.prayerflags.com/
"All the sublime teachings, so profound--to throw away one and then grab yet another will not bear even a single fruit. Persevere, therefore, in simply one."
--Dudjom Rinpoche, "Nectar for the Hearts of Fortunate Disciples. Song No. 8"