I think these days you can trade for specific Pokemon via Pokemon Home... otherwise you can ask around here or on a trading megathread on Reddit

Though fair warning, not all Pokemon can be used in Scarlet/Violet. You can only use the ones that are already in the Paldea Pokedex (see here, heads-up for spoilers), plus a few more (see here).

I think they're making more Pokemon compatible via DLC later? It's a mess lol