Monarch caterpillars are green and black. The milky sap sounds like a milkweed, but I don't recognize the purple. We live in very different parts of the country and you may well have a type that we don't.

If you lived in the western half of the state where it is quite dry, perhaps the milkweed you knew as a child is a native milkweed that grows in California (Asclepias cordifolia) aka Purple Milkweed or heart-leaf milkweed. I don't know if it grows where the conditions are more moist, such as might be found along the eastern part of Oregon.

Here is a description I found by searching: Asclepias cordifolia, Purple milkweed or Heart leafed milkweed is native to northern California, the Sierra Nevada mountains, Oregon, and western Nevada.

Google the proper name and see if the photos that come up in the search list look like what you remember. If so, perhaps you can also find a seed vendor by using google to search.

Mary