If you want a good seat you'll gonna have to pay for it, otherwise you'll have to be at the track earlier than everybody else go get into position, simple as that
I recommend the bus stop chicane in case you go for general admission. I had a gold seat at Eau Rouge last time I went, but walked around and explored where to best see the action while the Porsche cup and GP2 was going on. The path and hill alongside the chicane was a great place to see the cars in action. They arrive at great speed and brakes are visibly glowing red hot going into the corner. It's also a great overtaking spot, especially for GP2 where the number of overtakes usually is a bit higher. For F1 qualifying and the race it's often very crowed though, people climb all up onto the hillside and in the trees to see the cars.
Pouhon is nice as well. Very impressive to see the cars cornering at such high speed. However the action is very limited, cars are mostly coming through one by one. You're further away from the track and get less time to see each car in action compared to the bus stop chicane.
La Source I can't say a lot about, haven't been there. Eau Rouge is obviously massively impressive and you'll have a good view of the cars exiting La Source and building up speed all the way until they pass you. Visibility onto the kemmel straight is non existent though.
Given the available space, along the Kemmel straight might actually be the place you're looking for if it has to be cheap yet broadly accessible.
Anyway, enjoy your weekend. I hope you have a blast.