Bitter Vetch Availability?

Can I obtain this herb? And where can I get it?

There are several plants that called "bitter vetch". We assume you are referring to Lathyrus linifolius which has been in the news for its ability to suppress hunger. The tubers, described as having a "leathery licorice" taste, apparently have been used for centuries to fight hunger pangs, especially when crops fail and famine sets in.
Another "bitter vetch" is Vicia ervilia, a bitter lentil-like legume which was eaten by poor people during famines in Europe. The seeds need need to be boiled in several changes of water to remove the bitter taste.
We do not have either of these. But we can see why there would be interest in Lathyrus linifolius. We will look for it for possible inclusion in a future catalogue.