A buddy of mine and I are going to make a roadtrip through the United States, lasting eight weeks and starting out in New York City and leading us through various cities such as Miami and New Orleans to end in California.
We want to finance the trip with our savings.
Since the both of us are still in school right now our budget is not very high. Thus we plan to earn a little bit of money during the trip with product placements which are advertised in blogs and social media like Instagram.
Additionally we had the idea to put advertisements in form of stickers and decals from companies, which are willing to sponsor us, onto our car.

Now we are looking for potential sponsors and additional ideas / tips and ways to save and potentially earn money in the US with regular ESTA visas.

You are dead in the water - those traveling on ESTA visas are absolutely not allowed to work. Not even free/non-paid/volunteer work . . .

Check out the ESTA thing, I don't think it's a matter of "working", which you wouldn't really be doing, but whether you are allowed to earn money or not in any form, I guess. I suppose it's a matter of semantics whether being paid to carry advertising on you is really "working" or not. I don't really know, this kind of advertising seems to be around, though. I know if you get room and board in exchange for work, not cash, that's still working but those situations people really are working (like an au pair). I think you'd have to ask the authorities whether getting paid to post things on Instagram falls under that category.

in any case, have you thought about the fact that if you are renting a car, you cannot be defacing it in any way?

I agree working is out, but there are many ways to save money. Stay in hostels, even camp out, etc.

I thought you had to be a celebrity or have some following already on Instagram, etc., for a company to pay you for brand placements, so many followers or something at least. Do you have that? If not, I really don't think that is feasible, anyway, no company is going to pay someone for product placement who doesn't have an existing blog with a lot of followers that has a reason related to the product (like cosmetics on a beauty blog).

The main jobs I know that get paid under the table are manual labor around where I live. Not an easy way to earn some bucks.

The Visa Waiver Program forbids receiving any form of compensation which would also include stuff like room and board, using WOOOF. etc.

I’m curious if this ever worked out for you?

I'm pretty sure it was DOA.