I'm looking for marketing ideas on how I can locate EXISTING 457(f)plans(or "risk of forfeiture plans") with tax-exempt and other non-profit organizations. I understand they do not show up with 5500's and 990's. Any help would be greatly appreciated.

These plans are generally registered with the DOL as ERISA exempt "top-hat" plans. All such filings can be located free of charge at freeerisa.com. Most of the plans listed will not be 457(f) plans, however, where the sponsor is a tax-exempt organization, you can probably assume that it is.