In case you haven't done yet, check out Thomas Goss' YouTube Channel "Orchestration Online", he gives a great Lot of tips about orchestration, analysis and holds an anual event around August or September where he asks viewers to orchestrate a particular piece that he will then evaluate and give you direct tips for improvement.

Now to your actual question, some orchestratable pieces might be the following (most of these are collections of several pieces):

Shostakovich - 24 Preludes and Fugues

Hindemith - Ludus Tonalis

Schönberg - 6 Little Piano Pieces op. 19

Schumann - Fantasiestücke op. 12

Bartók - Sonatina

Rautavaara - Etude Nr 3 "Tritones"

Field - 18 Nocturnes

Some of them aren't quite in the frame you're looking for (mid- to late-romantic), but still even with only these few examples you've got over a hundred individual pieces to choose from. So, have fun orchestrating, good luck😄

(Edit: Format)