Just buy them from me. My forests go up slopes quite happily . I’ll dig out some photos of a Stonne game where the wooded slopes were around 45° or 1 in 1.
Here you go:
Großdeutschland IR emerging from woods on the Stonne escarpment. Note angle tree trunks make with the ground towards the bend in the road near Pain du Sucre
Enjoy your gardening
Les & Alison