You've got four main ways to get inbound liquidity.

1. Pay for it from a liquidity provider.

2. Some kind pleb noderunner opens a channel to you.

3. Send sats out of your channel and back to on-chain by using Boltz.exchange.

4. Build it up naturally as you spend your sats.

I generally go with option 3, unless you know someone personally who is willing to lock up a decent amount of sats into a channel with you.