I mean, like a hand pump.
How could I make a pump for self-inflation?
Not to be a dick man, but really, you can't scrape together $10 for a hand pump? I'm broke, but even as a kid in grade school, I could save lunch money for a couple days to get that. You're asking a lot of questions that lead me to the hunch that you're probably not old enough to be on this site...
I can see how someone might prefer to make their own but i'm not sure how they'd do it. A homemade pump could be tailor made for the purpose. There are no manufacturers out there making pumps specifically designed to inflate people so far as i know.
Super-fucking-cheap idea: bottle, pin. Poke hole in bottle, insert, squeeze bottle with thumb over hole, let go of hole, let the bottle refill, rinse and repeat.
A bottle with a valve would work to some extent but the stroke volume would be small. Otherwise there'd be blowback. Each stroke leads to more pressure inside you and the air pushing back more strongly.
I don't think body inflation is actually possible in real life, at least... not in the way its shown in cartoons. So no, I don't think you could make a pump for self inflation.
You can use any small bicycle pump and some aqaurium tubing. Get the soft silicone tubing... and if you want automated inflation get an adjustable aquarium pump. Email me if you'd like some other tips and such. I've been inflating for 20 years now. It's quite enjoyable!