Answer: The liquidity provider can be a company or an individual that quotes both a buy and a sell price in a financial instrument or commodity held in inventory, hoping to make a profit on the bid–ask spread. At the crypto market appeared instruments similar to the forex market https://forextraininggroup.com/role-liquidity-providers-currency-market/ .
To find a good liquidity provider follow 3 common steps:
1. Choose most popular, trust exchanges on https://coinmarketcap.com.
2. Contact them and clarify the terms of cooperation.
3. Contract with the most convinients and get started.
Pay attention to the API protocol: it should be Fix API if it os possible.

After this exchanges usually use aggregation module to get exactly liquidity from couple of providers. Such instruments available now, just google it:
https://www.soft-fx.com/products/liquidity-aggregator/
https://www.forexchief.com/about/aggregator/
https://www.centroidsol.com/technology/c-aggregation

In the search process try find couple of reserve providers for cases in wich someone can't continue with you.