In addition to the very good suggestions to either build your own tile server, or contact one of the several companies who provide commercial services based on OSM, you might also want to consider the Open Mapquest tiles, based on OSM. I believe they are currently encouraging everyone to use their tiles:

http://open.mapquest.com

Or perhaps CloudMade? (Details on the commercial services page above).

"Don't bother asking"? Alternatively, ask them here:
http://help.openstreetmap.org/