In the 10 years that we've owned a home, we have done lots of landscaping projects. One place that we have consistently used is Dragun's Landscape Supply. Dragun's offers quite a bit of variety of merchandise - mulch, pavers and retaining wall materials, gravel, topsoil, and more that I just can't think of right now.
An added bonus is that they are open 7 days a week, and many supply companies are not opened on Sundays (which we found out a few weeks ago when calling around for pricing on mulch!)
Pricing is competitive with the big box stores. Delivery has always been fairly quick, within a day or so.
I think their delivery fee is just a little steep ($30-40 for the things we have had delivered), but check in the local Val-pak mailing and the Clipper Magazine - usually Dragun's has coupons - either free delivery on a certain number of yards or $$ off each yard of mulch, etc.