There are several possibilities to practicing outside of the studio.. but none of them as good as using a proper dip nib of course.
You can of course just use a fine pen or pencil to practice letter forms. Leave the shading and that when you have a dip nib at home.
You can try the Akkerman pump pens. I have tried one but many years ago, and was put off at how messy, fiddly and the high need for a learning curve it requires. By the time I put on the nib, clean the feed, fiddle with the placement and feed and clean off the ink from my desk and table, I could have pulled out a dip nib and ink. But this was 2 years ago, maybe it has improved, though I maintain that a clean nib is necessary for clean lines and shading, so I don't know how much it could improve without that important point...
You can also try cheaper fountain pens such as the Noodlers Ahab. I have one and while it shades okay, remember it requires a lot of pressure to shade and thus will not develop a sensitive touch. Still, it's okay for making fancy letters outside of the studio.
If you have the money, you can consider a vintage flex fountain pen which costs like $70+, a Namiki Falcon which costs $144 or a Spencerian modified Falcon for $250+.. or a wet noodle at $500+? I have recently reviewed the Namiki Falcon and modified Namiki Falcon in the point of view of Spencerian here, hope it helps:
If you have the tinkering skills, I have heard of people putting a Nikko G in their Noodler's pen.. a quick google search will reveal the step by step. I have not tried this but am quite interested if anyone tries it!