I have purchased seeds for blue zinnias from two different sources. Neither of them are germinating. Where is a reputable place to purchase such seeds?

Welcome to the site!

Do not believe that zinnia's come in the color blue, lots of scammer out there.

Lots of reputable seed sources, I like Baker Creek https://www.rareseeds.com
Burpee
https://www.burpee.com

I'm sorry- there is no blue zinnia that I've seen, as RJ @crawgarden noted.
It's unfortunate the gardening industry has this problem in the online marketplaces- ebay, amazon, etc. Please make a complaint against the seller in the hopes we can all unite and make a difference before the problems grows any bigger.

Lots of really great zinnias, though, just buy them from real seed companies.

Here is a vast sample of the color range of zinnias, from a reputable supplier- but not a blue one out of their 54 choices...
https://www.johnnyseeds.com/fl...

They are all fake scams, just like the blue, black and rainbow rose seeds.

Photoshopped zinnia colors.

My favorite is always Swallowtail Garden Seeds, Over 126+ Zinnias to choose from and they offer bulk seeds if your seeding along side a driveway or something. They list the heights and everything as well as new cultivars. https://www.swallowtailgardens...