You may want to start by thinking about whether you have any special background or knowledge that suggests a legal system you might want to study. Are you fluent in a foreign language? Do you have a relevant undergraduate background—anthropology, say, or history? Is there some society that you are particularly interested in or knowledgeable about?

If none of that works, the following suggestions may help:

Possible Legal Systems and starting places for information

Australian Aboriginal Law

Adat—traditional Malay/Indonesian law

Pashtun Law

Early Indian Law

Ottoman Law: State, Society, and Law in Islam: Ottoman Law in Comparative Perspective by Haim Gerber

Anglo-Saxon Law

Germanic Law

Saxon Law

Salic Law

Visigothic Law (Has been done once, but could be done again and better)

Alaman Law

Danish Law: Scanian, Zealandic, Jutlandic

Swedish Law

Welsh Law:  Charles-Edward The Welsh Law

The Law of Criminals

Shia Law

Nuer Law, Evans-Pritchard

Early Chinese Law Codes:
T'ang: Wallace Johnson, The T'ang code.
Ch'in: A.F.P. Hulsewé, Remnants of Ch'in Law: An annotated translation of the Ch'in legal and administrative rules of the 3rd century B.C., diwscovered in Yun-meng Prefecture, Hu-pei Province, in 1975.
A.F.P. Hulsewé, Remnants of Han Law, Vol. I.

One place to look for stuff: Chicago-Kent Law Review, Vol. 71, 1995-6, "Ancient Rights and Wrongs Symposium on Ancient Law, Economics and Society."