JaneQuilter and Watson ...
I'll answer both of you ....
Most LQS' sell it.
For
Watson, I have never seen it at Len's, nor Fabricland, but not saying that other locations have it than where I have been.
JaneQuilter, I'm not in the States, so don't know about Joanne's etc. I have seen here on the QB that some have ordered it online from Dharma Trading, USA based.
Here's a Dharma link that tells you more ...
https://www.dharmatrading.com/chemicals/retayne.html
I have always washed the fabric once with colour catchers.
If they come out coloured, I may wash and second time, but most times I go straight to Retayne.
I run hot water into my washing machine and add the Retayne. Let it Swish. Then add the fabric.
I usually toss in a colour catcher, just as a monitor, but have never had one come out coloured. Swish til all is soaked and submerged. Then I close the lid and let it sit. Usually hours rather than the 20 minutes they say. When I am ready, I finish the wash cycle ... and dry.
Until now, I have never heard the Synthropol recommendation but I don't see the need.
Actually I see it as countering what you want to do ... like it said, expect some fading.