It takes practice, to figure out the right words, timing, and tone to take when contacting people. Keep working at it and you'll get there.
Several times, I had new providers tell me that the moment they read my initial text, they knew I was both serious and a good client.
To write those texts: Always politely ask if they're taking customers, make sure you read their ad in full, give them a name to call you by, use full sentences (with correct grammar and spelling), and call them by their assumed name.