The Atlantic did a write-up in December [0] about retatrutide availability and they named at least one well known vendor (who also sells tirzepatide, semaglutide, and all the other usual peptides). Not the cheapest vendor, but well known, does the batch lyophilization in the US, still takes credit cards, and gets pretty solid group test participation due to their popularity. And if you know how their pre-sales work, the price premium isn't too terrible, worth it for the test participation IMO.

If you want to know who else to avoid, join the Discord the vendor links to from their web page and ask around in the general discussion topics, people will share.

[0] https://archive.ph/n0daP