Foster care agencies are usually in dire need of foster parents. Many agencies are happy to accept single foster parents. While foster care is very challenging, it can bring many rewards.
There are approximately 500,000 children in foster care in the US at any given time. Many of those children will never be able to return to their birth families. Some will be available for adoption, others will stay in the system until they are 18 - when they will be left to their own devices without the supports that a family traditionally offers.
Single foster parenting is challenging. You have no one else to lean on when times get tough - and they will get tough. Children who have been abused or neglected, the vast majority of those in foster care, are often difficult. They may be angry, scared, depressed or numbed out. They may do anything from the typical "kid stuff" to actually hurting people or destroying things. Being a foster parent requires huge wells of patience and love. The rewards can be powerful and unexpected.
If you choose to be a foster parent, be sure to do your research. If there is more than one agency in your area, talk to parents who work with both agencies so you can get a feel for how they handle things. Connect with other foster parents online and in your area as soon as possible. Experience can be vital and when you can learn from those with more experience you will be more likely to have a successful foster care experience.
Be prepared for the agency to question you thoroughly about your marital status and your reasons for being a foster parent. While many agencies welcome single foster parents, they do need to protect themselves and the kids in their care by making sure that you have the right motivations and a good understanding of what you are getting into.
Foster care can be risky, so knowing what you are getting into and how to protect yourself is important. False accusations have harmed many well intentioned foster parents and there are things you can do to reduce your risks.

