Reddit mentions of Yours Kitchen Organic Dried Thai Chili (Chifa Red Chili) From My Farm (3.5 Ounce)

I have never been able to find them, despite looking frequently. I'm pretty sure I have tried Whole Foods, Market Basket, and Wegmans. I have also tried a number of Asian specialty markets and haven't found them there either. For what it's worth, I am not Thai or of Southeast Asian descent, so I wouldn't really be able to recognize these chilies solely by their appearance and definitely not if they were not labeled as spur chilies. To further complicate matters, I have seen the Thai name for them vary significantly. I believe they're referred to as "prik chee fa" chilis, which may help you in your search, but I'm not positive.

I do know that you do not want to use the typical bird/Thai chilies you can find fresh in the grocery store, though. Those are a lot smaller and a lot hotter than spur chilies.

The woman whose recipes I use for Thai food says that you can use guajillo peppers if you can't find spur chilies. She also has the following note on her curry paste recipe.

>Large dried chilies are mild, while the small ones are spicy, so you can control the curry paste’s spiciness by adding more or less of the small ones. If you can only find the small ones, make sure you remove all the seeds, and use about 7 for medium spiciness and up to 15 if you want to get some sweat going! In this video, I only had small ones, used about 13 chilies and yes, it was quite spicy! 

Here's the recipe, in case it's helpful! Maybe you could try a Hispanic market and ask them to help you find a medium sized mild chili?

I eventually gave up and bought them on Amazon. Here are the ones I bought. It seems like the farm that grows them sells them directly on Amazon, which is nice. That said, it did take a little over a month for them to arrive. It made a tasty chili jam, though!