Have you reached out to your local clan to see if anyone has a 3d printer? They might be willing to help you for less than a service.
I am not sure what helmet you are looking at, but Thingiverse.com has a lot of free ones. Just put "Mandalorian" in the search bar and you will have a lot of results to choose from.
For sizing, you can pay $30 for Armorsmith, a program that lets you create an avatar to your measurements and size pieces accordingly. If you have Windows 10, the free option is to right click on the file, go to "open with" and click 3d builder in the drop down. It is very simple compared to other 3d software, but you can split and size a file easily.
You have some other things to consider when having an object printed. I hear a lot of people charge by the hour, and with a low resolution, it will print quicker, but it will greatly increase your post production time in terms of sanding and priming to remove the print lines. I know some people buy their buckets on etsy. For reference, a 1kg roll of filament is about $20 and a helmet will use less than that.
I hope some of this was helpful. Good luck in your search.