they're not that expensive. limonene is sold by the quart online, and it's more expensive than white spirits, but totally reasonable price for hobbyists or domestic uses.

We can always use waste ressources, like eucalyptus oil that would be left as waste from the paper pulp industry (eucalyptus is a widely used plant for paper pulp, especially in southern europe where it has completely taken over portuguese tree plantations), or limonene from the citrus juice industry. Not saying it would be enough to replace all use of petroleum-based solvents, but it's a start.

Still waiting for limonene to finally replace white spirits in every hardware store. We have tons and tons of citrus peels from the juicing industry, i'd bet there's enough to produce a decent amount of more eco-friendly alternative solvents to white spirits, even if they're sold at a premium. A lot of people would be fine paying a bit more, if it means it's non-carcinogenic and smells like freaking lemons and not poison.