Mayor Marion Barry, who last week criticized the D.C. City Council for using "fiscal mirrors" to offset threatened cutbacks in welfare for the working poor, said yesterday he would try to find $4.5 million for that purpose in the current budget.

The council tentatively had approved a measure last week to maintain present levels of welfare payments for 3,500 working-poor families who otherwise would have been removed from public assistance by new Reagan administration guidelines.

Council member David A. Clarke (D-Ward 1), chief author of the plan, and council member Betty Ann Kane (D-At Large), insisted that up to $10 million could be squeezed from the current budget for the working poor and other purposes.

At that time, Barry said through a spokesman that while he opposed Reagan's cuts in funds for low-income families struggling to gain financial independence, "The council should not use fiscal mirrors in making its decisions."

Yesterday, Barry insisted that he and his budget experts never ruled out the possibility of finding additional funds in the fiscal 1982 budget for the working poor, but simply disagreed with Clarke's and Kane's analysis of where that money might come from.

Clarke's scheme to scrounge up the money includes delaying the hiring of additional staff for a corrections facilty and levying a tax on the computer industry.

"We're working to identify if it's possible to continue the assistance ," Barry told a reporter yesterday. "The kinds of money that Dave Clarke identified as real money for this purpose is not real hard cash. . . . We're looking for real money."

Barry raised the possibility he would support some type of plan to continue assistance for the working poor while taking part in a panel discussion of the impact of Reagan's budget and tax policies, held by the Democratic National Committee at the Washington Hilton.

In outlining ways the D.C. government was seeking to offset federal reductions in Aid to Families with Dependent Children, Barry said, "The council and myself are working diligently to try to find $4.5 million for the working poor on AFDC."

Later, Barry told a reporter that City Adminstrator Elijah B. Rogers and top officials of the D.C. Department of Human Services were meeting late yesterday to consider ways of continuing aid to the working poor. Barry declined to say how his administration would accomplish this.

Informed of Barry's statement, Clarke said he was pleased but would reserve judgment until he sees where Barry proposes to cut the budget to come up with the necessary funds.

"The purpose of my motion was not to get the money out of any particular place," Clarke said. "I'm just concerned about where it goes. If he has some other ideas, I'm willing to look at them."