Ratios for Individual Companies
You can lookup financial ratios for individual companies in many of our company financial databases such as D&B Hoovers and Capital IQ. Locate the company's profile and find the tab that includes the ratios.
If those databases don't include the ratio you're looking for, you can calculate it yourself using the underlying financial information. The Corporate Finance Institute has some guidance.
Ratios for Many Companies
Screening for Companies with Certain Ratios
If you want to create a list of companies that have a specific ratio (e.g. they have above a certain debt-to-equity ratio) you can do that in many sources. Some options include Capital IQ, FactSet, or Bloomberg.