Just fyi, the hat is called the Papal Tiara or Triple Tiara. I'm not sure you could buy one...but you may be able to make one, check this link for directions:
http://gauntlet.ucalgary.ca/story/3636
Here is some info on the hat...
The Papal Tiara, also known as the Triple Tiara, in Latin as the 'Triregnum', or in Italian as the 'Triregno',[1] (#wp-endnote_Triregno) is the three-tiered jewelled papal crown of Byzantine and Persian origin that is the symbol of the papacy. Papal Tiaras were worn by all popes from Pope Clement V to Pope Paul VI, who was crowned in 1963. Though Pope Paul VI abandoned the use of his own tiara during the Second Vatican Council, symbolically laying it on the altar of St. Peter's Basilica, he did not abolish its use, explicitly requiring in his 1975 Apostolic Constitution Romano Pontifici Eligendo that his successor be crowned.
However, his immediate successor, John Paul I, refused to be crowned, and John Paul II, who also declined a coronation, told the congregation at his Papal Inauguration Mass, immediately after John Paul I's sudden death, that it was "not the time" to be crowned, while disputing the claim by critics of the tiara that it represented a papal claim to temporal power.[2] (#wp-endnote_homily)
18th century Papal Tiara, the oldest surviving tiara in the papal collection.
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18th century Papal Tiara, the oldest surviving tiara in the papal collection.
Though not currently used as part of papal regalia, the continuing symbolism of the papal tiara is reflected in its use on the flag and coats of arms of the Holy See and the Vatican. For a millennium until the reign of Benedict XVI it had been one of the ornaments surrounding a Pope's personal coat of arms. In a controversial break with tradition, on the first version of Benedict's personal coat of arms the tiara was replaced with the papal mitre. The mitre contains three levels reminiscent of the three tiers on the papal tiara.[3] (#wp-endnote_arms) This was the first such use since Pope Paul VI prohibited the use of mitres and croziers on coats of arms in 1969.[4] (#wp-endnote_crozier) However the Vatican subsequently issued documents which contained Benedict's coat of arms showing a tiara, not a mitre. It is unclear whether the initial arms announced have been withdrawn, or whether it is proposed to use two alternate designs, a "personal" set of arms with a mitre, and an official "state" set of arms for Benedict with a tiara.