ebay initially but then I realized that I could do a better job cutting my own T-slots and doing my own facing on the machine I built.
For ease of set up, I wanted the t slots to line up perfectly with the X and Y axis. This was most easily achieved (for me) by cutting the slots on the machine where they would be used.
But, there is nothing wrong with starting with ebay t-slot plates like these:
https://www.ebay.com/itm/T-Slot-plat...0AAOSwAypZsf5H
it would not be too much trouble to bolt a few of these to an aluminum tooling plate and giving it a one over with a fly mill. They are already face milled on both sides. from my testing, they seemed accurate enough for most people.
If you need hyper precision, you can also use granite surface plates or optically flat breadboards. I suspect that most of use don't need that level of precision and could not measure it if they did though. I struggle to even get the same measurement twice with my calipers....
Also, search ebay for old cnc pallets, t slot mill tables, granite coordinate measuring machines, vibration isolation tables, aluminum breadboards, Haas fixture plates, granite surface plates, granite gantry, welding tables, machine bases, machine tables. ebay is a mass grave for old precision machine parts...