Spirit offers two co-branded credit cards through Bank of America. These are the only two options you have for earning Spirit points via credit card spending.
Spirit is not a direct transfer partner of any major rewards program. If you're looking for an alternative to Spirit credit cards and you want a lot more flexibility in how you can use your rewards, I recommend looking at the Capital One Venture & Spark Rewards program.
Miles earned with Capital One's cards have the same baseline value as Spirit points except that they can be redeemed for 1 cent in value towards any kind of travel. So not only can use you use them for Spirit flights, but you can also use them for any other airline, hotels, car rentals, parking fees, tolls — you name it.