How to get Promoting items

[Michelaar]

I got a question. Where can one find the earth seal and the heaven seal? I am at chapter 7 side quest, and I don't know how many chapters are left until Lyn's story ends.

[Jedi]

Lyn's story ends on chapter 10, and you don't get any promotion items aside from the Knight's Crest (that Wallace auto uses on anything lower than Normal difficulty) until the Eliwood/Hector story proper.

[Leona]

Earth Seals are kind of plentiful, and are only really useful if you don't have the proper promotion item for one of your units.

Heaven Seal 1 in Eliwood's story can be obtained in chapter 24, if Hawkeye is still alive at a certain point. I usually use this chance to promote Lyn, as she's usually been sitting at level 20 for a while.

The second Heaven Seal you get after Battle Before Dawn (chapter 26) and can use it either in the Gaiden chapter (provided you unlocked it) or the next chapter. Eliwood gets automatically promoted after chapter 27, you don't need an item for him unless you're playing Hector's story. The chapter numbers are a little different for that one, but it mostly stays the same.

[Moblin Major General]

Knight Crest in Ch.9, promotes Wallace on Normal, Hero Crest and Knight Crest in Eliwood's Chapter 16/Hector's Chapter 17, along with a guiding Ring, Elysian Whip, and Orion's Bolt Before Ch 19E/20H.

[Paper Jam]

You only get one promotion item (a Knight's Crest) during Lyn's tale, and if this is your first time playing FE7, you'll automatically use it on Wallace.

To say more would spoil the rest of the game for you; I'll assume unless you say otherwise that you would rather I didn't.