Leg XX's site is still a good source for good vs bad gear, as well as an extensive list of suppliers/websites.
http://larp.com/legioxx/bad.html
The "Hebron" Italic G, as Quintius Clavus mentioned, depending on where it was made and -when- can be good or mediocre. I happen to have an Albion Armorer's Hebron, made some 10 years ago. The helmet itself seems pretty much spot on. The cross-brace reinforce over the top of the helmet was "too perfect", so I removed the mounts (which were small screw bolts, obviously not correct), hammered/roughed up the edges of the guards and put them back on with copper rivets. I also had to add a chin-strap "anchor" to the bottom middle of the neck guard, which was easy to make with a piece of brass sheet and rod, and a rivet.
The ear guards / flanges are the only other problem, as apparently the actual helmet had a more "pointed" shape to them, rather than cut off straight.
Looking at the photos posted, it appears the top photo is a different build from the bottom helmet! The neck guard on the bottom appears different in shape compared to the top one. What is the 'correct' shape I don't know. (I only really use the helmet now and then as a try-on at school programs, very rarely do I wear it)
One of our members in Leg III got this type of helmet from I -think- Soul of the Warrior years ago, and it had the correct ear guards, but the cheek guards seemed a little odd.
In the end, you'll have to carefully compare known drawings and photos (ie "Roman Military Equipment", Bishop & Coulston; and here on RAT the imagebase if it's still available?) and decide what maker's helmet gets as close to what you feel is the closest match, as otherwise you'll have to consider modifying it yourself, or have someone custom make it, etc. which can be expensive.
Also, don't feel restricted to the Gallic H & F types. The Coolus types (I think now called Bugenen?) that are all brass, from places like Deepeeka and Danyial, are pretty darned good. No matter what you do, it's not going to be "100% super duper accurate" because 1. everything back then was made by hand, and no surviving Roman helmet is absolutely straight and perfect (ask yourself what is your accuracy tolerance? 1 inch off or 1 mm?) , 2. everyone's heads are different. Just like hat sizes today, there's a "standard" but they all don't exactly line up with each other between the standards in the world - Same with these manufactured helmets. Everything's made within a certain tolerance. Nature of the beast when it comes to mass-manufacturing to fit several different bodies without going insanely expensive and custom. 3. Just don't buy that Trooper helmet ;D
Either way, good luck.