Search DataTable by column for a specific row
I have two suggestions:


Trim ColValue once rather than every iteration through the loop.
Use a case-insensitive string compare rather than performing .ToLower() on two different strings on every loop iteration.


Result:

private string FindInT(DataTable dt, string ColName, string ColValue, string returnCol)
{
    ColValue = ColValue.Trim();
    foreach (DataRow row in dt.Rows)
    {
        if (string.Equals(
            row[ColName].ToString().Trim(),
            ColValue,
            StringComparison.InvariantCultureIgnoreCase))
        {
            return row[returnCol].ToString();
        }
    }
    return "";
}