Purchased in the Bandit's stolen underwear shop

#1 - Robin's Tattered Green Boxers [bought at the Bandit's shop] (50 Gold)

#3 - Selim's Chainmail Briefs [repair Selim's weapon shop] (100 Gold)

#7 - Valdo's Traditional Underwear [repair the dock next to the lake] (100 Gold, 30 Meat)

#9 - Hanth's Two-Toned Briefs [repair Hanth's forge] (150 Gold, 15 Meat)

#12 - Rafaelo's Thong [repair Rafaelo's workshop] (150 Gold, 15 Vegetables)

#13 - Arthur's Mankini [repair Arthur's library] (100 Gold, 15 Meat, 15 Vegetables)

#14 - Liam's Purple Briefs [repair Liam's tavern] (150 Gold, 30 Meat)

#17 - Giant Loyd's Red Briefs [repair Whellcum's mine] (100 Gold, 25 Meat, 25 Vegetables)

#19 - Horace's Ghostly Briefs [repair the bridge to Horace's house] (150 Gold, 30 Vegetables)

#21 - Arthur's Thong [finish Horace's 2nd affinity story] (150 Gold, 30 Vegetables)

#26 - Zack's "Floury" Briefs [rebuild Zack's bakery] (200 Gold, 30 Meat, 30 Vegetables)

#28 - Grant's Combat Briefs [beat Grant's tournament (quest)] (300 Gold, 50 Meat, 50 Vegetables)

#30 - Daddy Orc's Briefs [unlocked during Robin's time in the dungeon] (250 Gold, 25 Meat)

#31 - Rulius' Jockstrap [finish Astaroth's 2nd affinity story] (300 Gold, 50 Meat, 50 Vegetables)

#33 - Stone Underwear [unlocked after defeating Grant X] (350 Gold, 40 Meat, 40 Vegetables)

#34 - Grant's Rags [unlocked after defeating Grant X] (500 Gold, 50 Meat, 50 Vegetables)

#37 - Tyran's Cursed Chains [finish Tyran's 2nd affinity story] (3000 Gold, 100 Meat, 100 Vegetables)

Affinity Underwear

#4 - Hubert's Favorite Underwear [finish Hubert's affinity stories]

#5 - Orderic's Favorite Underwear [finish Orderic's affinity stories]

#6 - Gregor's Favorite Underwear [finish Gregor's affinity stories]

#8 - Valdo's Favorite Underwear [finish Valdo's affinity stories]

#11 - Hanth's Favorite Underwear [finish Hanth's affinity stories]

#16 - Ghart's Favorite Underwear [finish Ghart's affinity stories]

#18 - Giant Loyd's Favorite Underwear [finish Loyd's affinity stories]

#20 - Horace's Favorite Underwear [finish Horace's affinity stories]

#22 - Beton's Favorite Underwear [finish Beton's affinity stories]

#23 - The Twins' Favorite Underwear [finish the Twins' affinity stories]

#27 - Zack's Favorite Underwear [finish Zack's affinity stories]

#29 - General Grant's Favorite Underwear [finish Grant's affinity stories]

#32 - Astaroth's Favorite Underwear [finish Astaroth's affinity stories]

#38 - Tyran's Favorite Underwear [finish Tyran's affinity stories]

Randomly unlocked

#2 - General Grant's White Briefs [unlocked automatically in the tutorial for sniffing underwear]

#10 - Hanth's Blue Briefs [unlocked after Robin wakes up from his first encounter with a Dark Knight]

#15 - Robin's Swimsuit [unlocked after completely repairing and furnishing Robin's house]

#24 - Voyeur Santa's Briefs [Drink the holiday eggnog at Liam's Tavern] {The story/eggnog is accessible only in December, the underwear however can be viewed all the time after unlocking}

#25 - Bandit's Favorite Underwear [found in the Bandit's lair in Golden Sand's last section after beating the Bandit while collecting the King's statue piece] {item can only be collected when visiting the location as part of the story, the room cannot be accessed again afterwards}

#35 - Robin's Favorite Underwear - automatically unlocked and found in gallery after defeating the king

#36 - Communal Briefs - unlocked by entering Robin's house after owning at least 30 pairs of underwear 

#39 - Dragonscale Briefs [finish Dodoro the Dragon's side-quest]

For some reason after beating the game 36th underwear didn't automatically unlock is there any other way for me too get it?