I'll let others answer about the Cardarine. I consider it a no go since the original manufacturer of the drug stopped clinical trials on it since the animals on it started developing cancer.
en.wikipedia.org
As far as boldenone, I think the reason you're thinking of it for endurance because of the fact it can increase EPO through an increase in red blood cells. But, this is an attribute all AAS have, so it's really not specific to boldenone.
But, you could Equibolan, it's a precursor to a precursor of boldenone. Hi Tech is the only vendor I know who currently makes it. Here's an article about it
Equibolin from Hi-Tech Pharmaceuticals is the next new legal prohormone that goes from 1,4DHEA to 1,4Andro to Boldenone - with advanced delivery!
www.priceplow.com
And you can buy it here:
Hi Tech Equibolan