Seranos are a little less ubiquitous than jalapas, especially red ones. Which I'll never understand, because I think both taste much better ripe. Depending on your needs, a Fresno could also be a substitute, and I think people generally look for and therefore sell them in their ripened stage. YMMV
Awww the elusive Red Serrano's, like already mentioned....Ripe Serranos or jalapeños are not common to find....more like impossible, though you may find 1 or 2 in a bucket.
Ripe Fresnos are far easier to find than the other two, for me I'd prefer to use Serrano's any day over jalapeños for taste & heat (That level of heat)