we have hardwoods under our cabinets, the builder told us that was the proper way to do it so that if you ever change the footprint the floors are intact.

Take a cabinet door to lowes or Home Depot cabinet section. They will find you a match

In my current house the previous owner added a painted black wood toekick. Probably sounds awful but I don't even see it. It blends to look like a shadow under the cabinet.

I would try to match as best you can and not worry if it's not perfect. Only you will notice the details and will probably only think about it when you're close up scrubbing the floors. If you know the brand of your cabinets, you might be able to order the filler/base boards. Maybe something like this would work: https://www.build.com/sagehill-designs-aha8bm/s993410?uid=2444969

If it makes you feel any better, the toe kicks on my new cabinets aren't an exact match to the wood used in the cabinets. My cabinets are maple with a veneer but the toe kicks are painted MDF

It seems like a waste to get hardwood floors installed under kitchen cabinets, but the second-hand ones I bought didn't come with toekicks. Builder friend says I should be able to get matching wood to use as toekicks, but I doubt I'll find an exact match with the color. Maybe it doesn't need to be that precise though. Floors will be white oak, cabinets are light/medium-toned cherry stain on maple wood (I think). What would you do in this situation?