Stunning machine, great find! To double check what needle your machine takes check the needle list on ismacs.net, it's searchable by name of machine and name of manufacturer.
If Hellen Howes doesn't have the parts you need, I'd recommend keeping an eye on Etsy, eBay, and Tias.com's Relics section. Older shuttles come up rarely so it may take a while to find what you need.