It's very individual, obviously, but find out if the clinics in your area have in-house donor pools. I ended up having to use an agency, but in general, in-house donor pools makes things easier & you avoid what is generally a significant added expense to use an agency.
What factors are important to you? Some women travel long distances in order to cycle for less money. Others need to stay local to cycle. Are you hard to match? If so, you may need to consider an agency. The clinics you are looking at should be able to give you recommendations for agencies if they don't have an in-house pool. If you go the agency route, you may be able to find one that only works with local clinics & donors. I did that & it saved me money & stress b/c I didn't need to deal with travel expenses, etc. (I think this type of agency is in the significant minority, though, so be aware of that. Most have donors from all over & don't limit themselves.)
If you are willing to share your general location, other posters may have suggestions for clinics or agencies. Also, if you are willing to travel, that is a factor.