An easy, somewhat expensive (if you do not know the trick) entry point that is easily accessible to anyone is nexaph.com. You can pay full retail (quite expensive) or go to their Telegram (listed at the bottom of their page) and wait for a pre-sale, where it'll be a little above half the price on the web page.

Still more expensive than the rest of the random Chinese vendors, but the upshot is that participation rates are very good for Nexaph and so there's a lot of testing done -- especially for GLP1s. For example, the current batch of Tirzepatide 60mg will have a 3- or 4-vial COA done by Nexaph themselves, another 3-vial random sample tested by customers (but then compensated by Nexaph), and at least one and maybe two big group tests with 7+ vials doing a full range of mass/purity/endo/sterility testing.

I've not seen too many other vendors that get such a high participation rate. And even for this company, for non-GLP1 peptides it's still tested pretty well but not to the same extent.

Even at their expensive price point, you could buy a few kits (10 vials ea) and pay $1000 for a full suite of tests and still be into it about $80/vial total, where a vial is ~65mg and lasts most people at least a month. Do the math on that -- compared even to cheap compounded tirz it is a fraction of the cost. There is good reason why a lot of people are taking that route now.

And back to your original question - once you are on the Telegram group, ask around and people will invite you to other Telegram and Discord groups for various vendors.

Or go to glp1forum.com and a lot of the same vendors will have posts there with information on contacting them.