Hello Dennis,
In addition to Randy's site mentioned above, from whom I have bought some very nice helmets, here are some others I have bought from and been very pleased.
Helmut Weitze at Helmut Weitze Militärische Antiquitäten. If you are loaded, go there and buy a collection. I have never been able to buy a Pickelhaube as they are just so expensive, but I have bought quite a few Imperial uniforms, tons of insignia from him. A gold mine if you have the gold. https://www.weitze.net/
Sergio Semino at Kunst and Militaria. A true gentleman to deal with, one of the senior collectors in Europe. Some exceedingly rare items. http://www.militaria-online.de/
Peter Whamond at The Collector's Guild. Peter and I are close friends, but I can say without bias that he is one of the most honest and respected dealers in the market. Not a large turn-over for Imperial, but he has had some superb helmets roll through his shop. Several of which are now here! http://www.germanmilitaria.com/
Malcolm Fisher at Regimentals. Many collectors state he has somewhat of a shaky reputation for Third Reich items, but I have bought about 6 absolute gem Pickelhaubes from him. http://www.regimentals.co.uk/
Ken Greenfield at Der Rittmeister Militaria. Prices are too high for me for helmets, but I have bought quite a few caps and insignia from him. A gentleman who stands behind his product. http://www.derrittmeister.com/
A word of caution that applies to your money and ALL militaria dealers if I may; I have three 'rules' for collecting:
1. The seller's description means nothing. Repeat: The seller's description means NOTHING. Do not believe a seller's description of anything you are looking at. Use your eyes to look at the item, use references, use my web site, use this and other web sites, compare to originals, and decide for yourself WHAT it is and IF it is correct and original.
2. If it is not correct, do not rationalize "well they could have...." etc. Just do not buy it.
3. If you buy a low cost and low quality item, you will always have a low value item. If you spend a bit more and get good condition and all original parts, the value will escalate, and you will enjoy it more.
Good luck! Tony