You can go to replay.pokemonshowdown.com and find all your replays, be it a year or fifty years old
1. To search replays of your own account, type your account's name in the Username tab, and click on search, all your saved replays would be there.
2. Then comes "Private Replays". If you, or your opponent had clicked on the "Don't allow spectators" option before battling, a replay automatically becomes a Private Replay, which means that only you, and your opponent, would have direct access to it.
To access it, go here, enter username and password, click on the "Replays" in the bottom left of the screen, a new tab opens. When it opens, type your username in the Username tab, and when the page opens, there would be two tabs, one "Public", and the other, "Private". If you click on the Private tab, you would be able to see those replays. To share these, you would have to share the replay's link with your mates (like me xP).
3. Then comes replays according to format. Ig you alr know the different formats, like Gen 8 OU, etc. To see replays of said format only, type the format's name in the Format tab, and the most recent replays are shown at the top, etc. If you want to see the bigg boss higher class + rank replays, click on the "Highest Rated" tab, to see the nice 1900+ (sometimes) replays.
4. Then is, replay's according to a specific keyword. If you want to learn some new meta new strats, like of, say, Groudon-Primal, you can click on any of the two main searching options, "Username", or the "Format", WITHOUT filling out anything. This opens a new screen, where there would be the "Experimental full-text search" tab. You can search these specefic things here:
Items: If you search something like Choice Band in the Experimental full-text search, replay's in which the iteam was used would be displayed. Bear in mind that some items are revealed only through Frisk, or are when Knocked Off, and so all the replays with that item may not come, only those in which the item was "revealed".
Pokémon: Enter a Poké's name, like Groudon-primal, and get the most recent replays in which it was used.
Moves: Enter something like Perish Song, and get it's replays.
The last one is really helpful if you wanna learn new strats, etc, as replays on it are sometimes useful!
Also remember, it goes by the chat I guess, and so if some random guy comes in a battle and types "moonblost frem scerf xerneas ohko yvultal", that replay would also come if you search Xerneas.
So don't watch those :O
Source: I go there for fun almost always, and uh.. I go there :p
Hope it helped!