Options for advertising online are limitless. It is entirely possible to advertise in dozens of places without ever spending any money.
Most of the free advertising sites also have the option to purchase ad credits. There are still others that are paid services only.
My advice is, only pay for advertising on sites that you have tested for free and know they get results. If you are tracking your links, you will know which sites are getting clicks through to your pages. Take traffic exchanges as an example.
All but a handful offer free memberships. The free ones all have options to upgrade your membership or make a one time purchase of credits. It would not make sense to spend a ton of money buying credits at multiple sites if you had not tested them to see where your ads are converting the best.