Mostly blue = most desirable = most expensive. If that's what you're after and it's only this one rock (regularly collect? new to collecting?) then I would go all in and splurge on one large chunk.
I would definitely avoid the "cheap chinese seller" route. Plenty of dealers sell it - just make sure they are reputable. Minfind has lots of reputable dealers but it's expensive. Dealers mean more hand exchanges which means higher costs since everyone wants their cut.
As someone else suggested - go to a show. Shows have great prices, and even better prices on the last day when dealers are packing up. You can also see it in person which is always ideal.
E-rocks is a great auction site with reasonable prices and rare stuff. You can also set up your account when new lapis is added. The positive is that lapis is fairly common so you might avoid crazy bidding wars.
Instagram is another where folks are selling (with paypal) because there's no seller fees (unlike ebay/etc). There's a lot of international sellers on IG.
Overall keep in mind that ONLY buying from the US means you will pay more because someone else has done all the work for you and paid the shipping/fees/etc. If price is a consideration for you then you should absolutely open your search query to a direct overseas deal - I would search instagram for direct sellers.