Pokemon Scarlet and Violet

Number Ribbon How to get
Paldea Champion Ribbon Become the Paldea Champion and enter the Paldea Hall of Fame
Once-in-a-Lifetime Ribbon Obtain on a Pokemon via Surprise Trade (1 in 100 chance)
Hisui Ribbon Available only on Pokemon that participated in the Photo Studio in Pokemon Legends Arceus

Big thanks to u/CaptainTimey on Reddit for the help with the ribbons and their locations!

Marks

You cannot obtain all of these marks on a single Pokemon. These can only be found on wild Pokemon.

Number Mark How to get Chance
Destiny Mark Found on wild Pokémon during the date registered as the player's birthday. 1/25 on player's birthday. Unobtainable on all other dates or if no birthday has been set
Jumbo Mark Speak to the Hiker near the Mesagoza (West) Pokémon Center. Must have a scale of 255 1/16512 - Base rate
1/128 Humungo Power Lv. 1
1/96 Humungo Power Lv. 2
1/64 Humungo Power Lv. 3
Mini Mark Speak to the Hiker near the Mesagoza (West) Pokémon Center. Must have a scale of 0 1/16512 - Base rate
1/128 Teensy Power Lv. 1
1/96 Teensy Power Lv. 2
1/64 Teensy Power Lv. 3
Itemfinder Mark Currently unobtainable. According to Bulbapedia, this is likely due to an oversight. Unobtainable
Partner Mark Granted to party Pokémon (with friendship of 200 or above) with player steps. (Does not require them to be walking outside of their Poké Ball) Random
Gourmand Mark Can be found on party Pokémon after making a sandwich when any Meal Power is applied. Random
Alpha Mark Transfer an alpha Pokémon from Legends: Arceus via HOME Guaranteed
Mightiest Mark Pokémon caught during 7★ Tera Raid Battle events Guaranteed
Titan Mark Found on former Titan Pokémon that reappear after obtaining their Titan Badge. Guaranteed

Big thanks to Bulbapedia for the information!