There is a lot of information here that can get you started. Use the search tool to find progress shots/tutorials. I don't think anyone is really going to be able to help you out with anything until you get started. Look around the forums for a while and read up. Good luck.
As mentioned use the search to read up on the "how tos' " in building a helmet.Or keep an eye on the Cargo Hold to see if any has a helmet for sale.Most of the time there wil be a Boba helmet but you could get one make a Jango lid.There is ever now and then a Jango lid up for sale.
If you are wanting to just buy one already made be prepared to spend some money.It might be lower cost to get a Boba lid and turn it into a Jango lid.
An another factor is the quality of the helmet.In my opinion a Rubies or DP is going to be at the low end of quality compared to some of the higher cost.
Back to your question of helping you make a Jango helmet...do a search here on "WoF helmet templates"."Wizard of Flight"(a memeber here) did a fantastic job posting templates on making a boba helmet.With very little modifing you can make a Jango helmet.Then do a search here on painting the helmet.
thats just my .02