Avete!
Manning and Lonely Mountain Forge are both custom armorers, so they should be able to make what you want, provided you give them good instructions. That helmet shown by LMF is kind of squat in shape, but I know he's done nicer ones.
The first Deepeeka one seems to be based on one of the helmets found at Xanten. The crest knob is unusual in that the sides of the cone have been sort of cut back so what you're left with is like 2 triangles rising from a disc. It was NOT found with cheekpieces, by the way, though Coolus C cheekpieces were found loose in the same area. My guess is that this helmet was really a Coolus, not a Montefortino at all. The crest knob is certainly applied, at least. I saw one of these repros a while back, and what the Indians had done was cut a spun dome in half from side to side, and welded in a strip between them. This was to make the bowl more head-shaped, but I think they overdid it, so it ended up too long...
The second Deepeeka Montefortino looks great in the photos! But it turns out to be huge, so unless you have a VERY large head (over 24" circumference), it's going to look like a bucket. Too bad!
The crest knob on the Albion Montefortino isn't like any Roman example I've ever seen, which is probably why they market it as Celtic. It seems to be copper rather than bronze. You might be able to file the knob into a more common shape, maybe even tin the whole helmet, if the color is too coppery for you. Is Albion still offering this? They've dropped a lot of their Indian-made helmets recently. This one is by International Steelcrafts, not Deepeeka.
My own Montefortino is more like the Buggenum example, on the right in this photo:
http://www.larp.com/legioxx/Mont1.jpg
I made it from a spun bowl, with the knob from an old porch light welded on top! No, you can't have it!
My advice would be a custom piece by Manning, I think. Just get one like that brass one but without the thick cabled rim. Make sure he leaves the surface all irregular with hammer marks, and uses only one rivet in each cheekpiece hinge, for more accuracy! It still won't be cheap, though. Or get a spun dome and weld the neckguard and knob onto it--not exactly a fun job.
Same old song, the Indians are SO CLOSE to making something great! It should be easier.
Good luck and Vale,
Matthew/Quintus