It depends on whether the third party manufacturer allows for single item orders versus bulk. They might only take orders of a minimum number. If you can't pay for that many at once, you'll be out of luck in getting whatever printed or made up for resale. It'd require capital immediately. Although you might be able to haggle a line of credit. Maybe.
I'd research that one first before actually getting into it.
Switching out to making your own merch also requires immediate capital. You've got to figure out where you can buy your materials and if it's clothing. How you're going to go about printing say a piece of artwork on it etc. If you've got a printing press or machine. You'll be fine, if not than it'll be more difficult.
It might be possible to get the third party stuff done cheap locally if you know any businesses that specialise in say printing stuff on clothes. I'd check it out before doing anything. Might be more economical than doing that stuff online. Keep your options open.