To make the stretcher bars, I start by gluing a delicate piece of 8' glass bead molding onto the edge of a 2"x2"x8' piece of pine. I typically buy four 8-foot pieces of molding and four 2x2x8 lengths of pine at once. More recently I've been buying redwood, as the pieces are milled a little straighter.
The fine molding is available at Lowes: EverTrue 0.375-in x 0.375-in x 8-ft Interior Stain Grade Radiata Pine Glass Bead Moulding (Pattern 148), Item #: 13181.
I used to use a fine quarter-round molding from Home Depot, before the Lowes opened in Vallejo.
Once the wood glue is dry, I cut pieces to the desired length at a 45-degree angle using my Ryobi chop saw.
To put the pieces together, I used to use two DeWalt drills (one for drilling pilot holes, and one for screwing the wood together). The old drill lost its mojo, and I'm down to one drill. That's fine, though--I'm using wood screws that don't require pilot holes.