AC is a generic term. It applies to a variety of circuits that range from signal levels to power circuits. Voltage levels and current levels can be just as diverse DC - and it is the range of these that you want to explore that will dictate the sort of solutions you will need to consider.
The one parameter common to AC circuits (that you don't get with DC) is frequency. It gets interesting when there is more than one frequency involved.
An AC source can provide a negative current for op amps etc. can't it? I can convert the - side to -DC with an RC? Or am I missing the point?
I won't say you are missing the point - you have basically got it right - but there is a little more to it.
All voltages are expressed as a value
referenced to a specific point. This is often called the 'ground' - but that is, strictly speaking, misleading. It's just the zero volt reference point.
To have 'positive' voltage and 'negative' voltage, you need 1 reference point, one 'positive' point and 1 'negative' point. 3 points in total. The typical way to achieve this from a mains supply, is to have a transformer with a centre-tapped secondary or one with two identical secondaries. Wired appropriately, with the rectification circuits, these produce the 3 points you need.
With a single secondary with only 2 wires, you can still create positive and negative power rails - but you have to create your zero volt reference point ... and there are a few ways of doing that.
The most important thing to remember about 'positive' and 'negative' voltage is that the ONLY difference is where you put your meter leads! The black one is typically on the 0V reference point - and that's the ONLY difference in this diagram: