According to
Thief Training Reference - MapleWiki.
Thief Leveling AreasExp Areas10~15 Henesys Hunting Grounds (Snail / Slime / Orange Mushroom)
10~15 The Tree That Rose , Dungeon, Southern Forest I (Slime)
10~20 Mushroom Garden (Shroom / Red Snail / Orange Mushroom)
16~21 Pig Beach (Pig / Ribbon Pig)
17~30 Transfer Area (Stirge)
18~22 Kerning City Construction Site , Damp Forest (Octopus)
20~25 B1 (Bubbling)
20~30 Ant Tunnel I (Horned Mushroom), Kerning City Party Quest(assorted monsters, 200- 800 exp after each stage).
Money Areas10~13 Henesys Hunting Grounds (Blue Snail / Slime / Orange Mushroom / Red Snail / Shroom / Stump)
13~18 The Tree That Rose , Dungeon, Southern Forest I (Slime)
18~25 Pig Beach (Pig / Ribbon Pig)
20~30 Ant Tunnel I (Horned Mushroom)
21~30 Kerning City Party Quest (Assorted Monsters / Tasks)
25~30 Orbis Tower Floors 18~20 (Jr. Sentinel) Singapore, Central Way 1 and 2
Good Meso Drop