TBH I would put more money on Baltasar Carlos surviving (it was an attack of appendicitis, that the Spanish tried exorcising him for. Get a French doctor who knows what he's doing to treat him instead, and we might go somewhere), Granted, he had a short-lived/stillborn full-brother, Francisco Fernando (b&d 12 March 1634), so you could go that direction too. Baltasar married Marianna (I started a TL where her epilepsy kills her en route to Spain, although BC is hale and hearty), which puts her out of Felipe's reach. Felipe needs to find another bride.
The only real options I can think of in the 1640s/early1650s to marry him would be Austrian or Italian. Bavaria had no one available, the Princess of Poland might fly but considering how she went through betrothals OTL before finally marrying relatively late, I don't think the Spanish will be happy. Now the Further Austrian line are only Felipe's first cousins once removed, and there are ladies from Parma, Modena, Tuscany (Anna de Medici married pretty late and still managed 3 kids), Mantua and Savoy. If he's got a surviving son and heir and simply wants to shore up the succession, then he's not getting a royal daughter, and Anna de Medici or such would be a good idea (I went with Anna de Medici in my TL, and married her OTL husband to the OTL Empress Eleanora Gonzaga Jnr in a double match between Austria and Mantua)