You can review:
adoptivefamilies.com
It has all of the links to answer all of your questions, agencies, costs and all, and if you have any thet you don't find, you can e-mail them and they will answer them for you. Check out some local adoptive meetings. Agencies provide them free, but all urban areas have seminars with many agencies represented, and experts in all aspects of adoption (for a reasonable cost, and they usually last a full day or two). They're really fun and informative. You can select the sessions that you want to explore.
Your first decision is whether to adopt domestically or internationally. Believe it or not, they both will cost about the same in the end. Regardless of where the child is adopted from, the US government offers a tax credit of up to $13,170 for qualified expenses paid to adopt each eligible child. This will cover about 25% or more, of the cost, or lots more if you adopt a foster child or special needs child. The next will be the age child. An older child will be more available than a baby. Toddlers are great! One bit of advice is, if possible, adopt a child younger than your son. It is more natural for the family. In any case, it would be a blessing and change the life of that child to have you for parents! Don't wait too long, because if you decide to adopt internationally, most countries have age limits for adoptive parents for normal healthy kids.