The Selphie Triple Triad card is, naturally, a Level 10 Player Card. Out of all the six main playable characters', hers is listed first in the inventory, standing between the Laguna and Quistis cards. Its values are A (top), 8 (right), 6 (bottom) and 4 (left), making it decently well-balanced for playing. It holds no elemental property.
Since Player Cards are usually won from people who possess a close connection to the character in question, it makes sense that the Selphie card can be obtained from a friend of hers back in her one true home: Trabia Garden. At any time during or after your first visit to Trabia in Disc 2 (obviously excluding Disc 4, where nearly all locations are sealed), simply challenge the girl sitting near the gargoyle statue in the Front Gate area, next to a Thundaga draw point; she will eventually play the Selphie card, which you can win from her.
Through Quezacotl's Card Mod ability, the Selphie card can be refined into 3x Elem Guard, a relatively rare item. In turn, each of these can either be refined into 4x Elixir (removes all bad statuses and fully restores HP to one ally) through Doomtrain's Forbid Med-RF ability or used to teach a GF the Elem-Defx4 ability. I strongly recommend the latter option, since Elixirs are not rare items (they can be obtained through plenty of different ways, from refinement to stealing or even buying), while the Elem-Defx4 ability is extremely useful and worth having.
Under the condition that you have completed the CC Group Quest before the end of Disc 3, its members will be stationed on the Ragnarok in Disc 4 and willingly play cards with you: the good news are that if you had modded a rare card, you can now win it back from them (yes, any of them, although the left girl of the Diamond Duo is the best option, since she always plays the first rare card missing in your inventory). Note that you can do this indefinitely (mod card, win card back, mod card, etc.), which allows you to stock up on many rare items.