Yes, there are Z390 motherboards available.
Yes, there are Z390 motherboards available.
Simply browse the used market and be prepared to discover affordable options. Focus on the lower tiers rather than the very cheapest models, but avoid the worst-end-of-the-line boards like the z390a pro or z390 ud. Anything beyond that is likely a waste since the CPU overclocking has stopped. The 9th generation marked the end of viable performance, though a slight boost to around 5GHz might still help if you manage to find a better-end board for only a small price increase. Consider features and connectivity when deciding.