Goldenrod crab spiders (
Misumena vatia) aren't usually bred and distributed like many other larger arachnids are. When it comes to keeping them, their status is similar to that of many jumping spiders and orb weavers in that they're beautiful species, but they're not usually distributed professionally, and only a few private breeders may be keeping them in large numbers. You may have to go out and collect your own.
Their care is pretty simple though. A small jar will house one easily. You'll want a branch or flower in there for it to sit on and stalk its prey from. It will actually be easier for the spider to catch its prey if the enclosure is not too large. The spider should spend most of its time sitting in the same general area, such as on the flower. The smaller the enclosure, the less places there are for the prey to escape to. If you want a really nice looking enclosure, you may want to get a small vivarium of some sort (or make your own) and actually plant a live flower in it.
To feed it, you may want to use a pair of forceps and hold the prey item for the spider, though you can certainly try leting the prey loose to and see if the spider can catch it.
Good luck in finding some
If you actually find someone who breeds and distributes them, let me know!
