Senate opponents of President Reagan's request for $100 million in aid to the contras fighting Nicaragua's Sandinista government are discussing a filibuster as a way to stall the proposal, congressional sources said yesterday.

Several senators and their aides said that about 15 to 17 senators have indicated tentative willingness to join the effort. However, Sen. Alan Cranston (D-Calif.) cautioned that the idea is in the discussion stage and that "no firm decision" has been made.

Staff sources said the idea originated with Sen. John F. Kerry (D-Mass.) and was discussed at a meeting Tuesday in the office of Sen. Edward M. Kennedy (D-Mass.). They added that about 14 Democrats and one Republican, Sen. Lowell P. Weicker Jr. (Conn.), attended to discuss the pros and cons of what participants insisted on calling "an extended debate" rather than a filibuster.

"I don't think anyone really thinks that the contra aid bill can be defeated," said one participant who asked not to identified. "Instead, the idea is not to have a traditional filibuster where people read from the phone book but to make one last effort to educate the public about this issue through substantive discussion of the dangers that we see in aiding the contras."

Reagan won a major victory last month when the House voted to provide $70 million in military aid and $30 million in other supplies for the contras, or counterrevolutionaries.

The Senate, which earlier had adopted a different contra aid bill, now must vote on accepting the House version. Senate Majority Leader Robert J. Dole (R-Kan.) has not indicated when he will schedule the vote.

The Senate adopted its earlier contra aid bill last March, 53 to 47. If a filibuster occurs when the new version comes before the full Senate, Dole would have to muster 60 votes to choke it off.