Hardy Lavender (Lavandula vera)?

I have an outdoor herb garden that has been through 2 growing seasons (I'm still a neophyte). In the past I've chosen not some so hardy varieties that either don't grow very well or are killed by severe winter weather. Thus I am inclined toward finding the hardiest varieties. I have read that Lavendula vera is the hardiest Lavendar variety, but you don't carry it. Why not? My Munstead variety is not doing well, and may not survive this winter, mild as it has been.

Lavandula vera is an old botanical name, now replaced by Lavandula angustifolia. Your Munstead lavender is a variety of L. angustifolia, so in fact, you have had the species all along. Richters carries a good selection of L. angustifolia cultivars.
Hardiness is dependent on many factors, not simply the minimum winter temperatures. Where there is a good snow cover, for example, it is often possible to get plants to survive cold temperatures which they wouldn't ordinarily survive in areas where weather is milder over winter.
Lavenders require excellent drainage. If your plants are in a wet area or in a porrly draining soil such as clay, then lavenders will suffer root rot. Lavenders also prefer a neutral to slightly alkaline pH; so if your soil is acidic, you need to add lime to raise the pH.

Also, I am looking for Milk Thistle, which you seem not to carry. Any plans to add this to your inventory?

Yes, we carry milk thistle. It is listed under "T" for thistle.