United States

Your local sheet metal shop
If you bring them a good design they can likely fabricate it for you. See here for some designs.

Rain Harvesting
Manufacturer of a simple, relatively self-cleaning Leaf Eater and Leaf Beater rainhead filters designed to filter large debris before water gets to the cistern. Can be used for above and below ground tanks. Other filters and gutter screens are also available. In the US, these rainhead screens are also sold by Texas Rainwater Tanks and Loomis Tanks.
www.rainharvesting.com (US)
www.rainharvesting.com.au (Australia)

Rain Tank Depot
www.raintankdepot.com

Rain Technologies
Manufacturers of award-winning, sustainable products for collecting, storing and using rainwater. These innovative products include those for gutter filtration, as well as subsurface mass storage chambers. RainTube is Cradle to Cradle certified and NAHB GREEN PRODUCT approved, producing LEED points in several categories.
www.raintechnologies.com

Resource Conservation Technology, Inc
Supplier of a number of well-made, relatively self-cleaning German filters meant to filter large debris before water gets to the cistern. These are primarily meant for underground installation.
www.conservationtechnology.com

Silvan H2O
Manufacturer of Lazy Lizard final stage filter. The Lazy Lizard is typically used after a rainhead screen with a 0.9mm mesh. The Lazy Lizard helps further prevent small debris from clogging pump systems and RainBank-type systems that switch water feed between rainwater tank and municipal supplies.
www.silvanh2o.com.au

Arizona

Prescott
High Desert Rain Catchment
www.HighDesertRain.com

Tucson
Loomis Tanks
They carry RainHarvesting brand Leaf Eater and Leaf Beater rainhead screens.
www.loomistank.com

Quick Custom Metals
They can fabricate metal rainhead screens.
(520) 888-7313

California

WhollyH2O
A clearinghouse-in-progress of all California service providers and California (and beyond) products for rainwater, greywater, storm water and black water for the residential, commercial, industrial, institutional and agricultural sectors. This includes xeriscaping and water-focused landscaping products and service providers.
www.whollyh2o.org

New Mexico

WaterMart of New Mexico, Inc.
www.watermart.us