Clean Water Components is an on-line store selling kits for greywater, rainwater, and composting toilet systems. On the site are downloadable pdfs of the systems with all the parts needed to build them.
Check your local salvage yard! PVC pipe and fittings and flexible tubing can often be obtained cheaply, and reusing plastic pipes greatly reduces your system's environmental impact.
Craigslist can be a good place to find food-grade drums and recycled plumbing materials.
For laundry to landscape or pumped systems:
Kits: Clean Water Components sells a kit for laundry to landscape systems.
Fittings: Local irrigation and hardware stores carry fittings for PVC pipe and HDPE tubing. In urban areas, 1" barbed fittings for HDPE pipe can be hard to find. The Drip Works online store stocks fittings and the "greenback" ball valves, which resist clogging.
Tubing: 1" HDPE (high density polyethylene) tubing or "poly pipe" (polyethylene), or PVC pipe can be used where the washing machine's internal pump or an effluent pump distributes water. PVC systems are cheaper but harder to modify. PVC pipe is readily available from hardware stores, but 1" HDPE or polyethylene tubing can be hard to find in urban areas. In California, Ewing Irrigation carries 500 ft. rolls. There are several online places to order, one supplier is Pioneer MidWest.
1" brass 3-way valve: local plumbing supply houses, in Berkeley try Ashby Plumbing; online from Oasis Design.
For branched drain systems:
1 1/2" to 2" plastic 3-way valve: from pool and spa supply stores, Clean Water Components or Oasis Design. Common brands are the "Jandy" and "Pentair."
The rest of the plumbing parts for this system are standard 1 1/2" or 2" drainage fittings and pipe. Use "long sweeps" to make a more gradual turn that reduces clogging.
Flow Splitters: While you can use tee fittings to split a pipe's flow approximately, a double ell fitting splits the flow evenly by reducing turbulence inside the fitting. Allow a 2' run of pipe between the double ell and any upstream bends. You can buy "double ells" from a larger plumbing supply house, Clean Water Components, or from Oasis Design. You can insert a cleanout yourself. See Brad Landcaster's explanation here.
For drum with effluent pump systems:
Surge tank: Craigslist,
Effluent pump: hardware store
Pipe and tubing: see laundry to landscape or pumped systems above.
For constructed wetlands:
pond liner: irrigation store
bathtub: salvage yard, craigs list
stock tank: agricultural supply stores
gravel: Craigslist, soil supply yard (gravel is mined, in CA it's mined from river beds and is very damaging, so use sparingly)
wetland plants: local nursery, roadside ditches, friends' constructed wetlands.