Where tiaras come from all depends on the family. In Spain, they did not give Letizia a tiara when she married Felipe, but she has access to two of the queen's smaller tiaras- the floral motif tiara and the Prussian Diamond tiara. Queen Rania didn't receive her own tiara until earlier this year (unless she actually owned that emerald one)- the tiara she has worn most was originally owned by Queen Alia, then by Queen Alia's daughter Princess Haya.
In Scandinavia, Crown Princesses Mette-Marit and Mary were both given "new" (new to them) tiaras on their wedding days. They also received other tiaras. MM was given/lent the amethyst tiara, and Mary was given/lent Queen Ingrid's Ruby Parure (bequethed to Frederik upon the Queen's death, it was to be used by his wife; however, upon their death it reverts to the crown)
In between is Crown Princess Maxima, who has never actually had her own tiara purchased for her but, due to the nature of the Dutch royal jewels, has had free reign adapting existing tiaras to her own desire. Her wedding tiara used the base of the Pearl Button Tiara, and the diamond stars from another tiara. She has also adapted the Emerald Parure tiara, the Diamond Festoon tiara, and the Antique Pearl tiara. Maxima has worn the most extensive amounts of tiaras, but then again, she belongs to the royal family with the more extensive collection. She has worn, from what I remember: the diamond stars tiara (her wedding tiara), the antique pearl button tiara, the emerald parure tiara, the Mellerio ruby parure tiara, the the diamond festoon tiara, the aquamarine tiara, the Sunburst tiara and the diamond bandeau tiara.