Your main question is not that easy to answer......a reputable militaria dealer would be first and one which specializes in pickelhaubes is Age of Kings . They are based in Tx and have a website. Other sources would be militaria shows or EBay but you have to have knowledge and experience so as not to get burned by fakes. If potentially buying off the internet, post pictures here and our members will help out.
Value depends on condition, a decent Prussian one ....... approx $650.
Original straps have sold on Ebay for $250. There are fakes out there, and again best post pictures here and get some expert opinions. The leather should look old and stressed, most are hand stitched, some are riveted. The leather is only dyed on one side. The fittings also have certain characteristics as well. Before buying, hang out here read posts and ask questions, buy some reference books which are listed on the forum. We also have wanted and for sale sections on this forum where members can buy and sell.