1. Gold/Gem mines. If that's not an option, get some off the world market. However, Sierra Nevada in California should have a massive amount of gold shortly into the game.
2. Sell more stuff to your population. It's not about having more exports than imports in this game. You need to BUY everything your population will buy. You'll break even from reselling it and then gain state funds in taxation (corporate/excise taxes). Also, when you get the chance, put trade fleets in the biggest MTB's. You get maritime tax for the amount of trade done there, and I believe it's based on your commerce value present as well.
3. Build factories that are vastly profitable. Luxury goods and Manufactured goods will almost always sell and will count towards that maritime tax.
Edit: I realized after writing this that you were asking how much is too high. Sorry.

It's not about having high tax rates, it's about the volume of stuff that you tax. The census and corporate taxes are the ones you need to watch the most. You'll almost always take contentment hits from census, but the higher it is, the more you take. I prefer to keep it at the half-mark or less, and only move it up as needed. Same with the corporate tax - this one is a little easier to mess around with, but it still tends to make people mad. I don't feel like tariffs are a good way to go - you want to find the point that it's giving you the most internal consumption that you can get (on the f4 screen - check the demand numbers), especially as the US since your demand will be low due to a low population. Also, generally, you want to aim for a contentment raise of AT LEAST 0.40% a turn. That should shield you from too much contentment loss.