In the UK, you can buy perennial meadow seed mixes from https://www.scotiaseeds.co.uk , https://wildseed.co.uk , https://www.wildflower.co.uk, and several other companies.
Elsewhere, there should be equivalents.
While this works, an alternative if you have a lawn already is to just stop cutting it every x weeks but mow once or twice a year. Mow paths where needed. More often than not there are already seeds/plants in the soil which just never got the chance to grow. And if not, depending on where you live, fauna and wind will transport seeds to your garden. The end result (mind you, takes years) will usually be similar: of the seed mixes bought, a bunch won't grow, others might but won't thrive. Whereas starting from 'scratch' this also happens. And what's left are those which happen to thrive best on the local soil type and circumstances, which usually is what works best for the local ecosystem.