This is somewhat confusing, are you looking for a managed server with cPanel/WHM from which you can offer shared hosting and reseller hosting to your own customers? Do you expect the said dedicated server provider to offer support for your customers as well?
High Bandwidth Servers
Custom Hosting Solutions
Basically, as i understand you need a web hosting company who provides different kind of hosting packages which includes shared hosting, reseller hosting and dedicated servers which you can resell to your clients. And they should also manage your clients in terms with support.