Dealers I can think of are Helmut Weitze in Germany, Militaria-Online.DE, also in Germany, Saxonia Militaria in the U.K., Grenadier Military Antiques in the USA. Many other dealers whose primary focus is Third Reich items, also have Imperial headgear for sale.
Weitze and Militaria Online have great selections; not cheap, but not outrageous, like Der Rittmeister here in the US. Regimentals in the UK has a good selection, but opinions on them are mixed at best. There are also usually a decent number of Pickelhaubes for sale on Ebay Germany, and you may get lucky.
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