Alternative Widget to RelatedItemsWidget

Is there an alternative widget for RelationChoice and RelationLIst fields? Something simple like AjaxSelectWidget? We tried using the AjaxSelectWidget with custom data converters but we encountered some problems due to our unfamiliarity of its requirements.

A link to source code for a simplified version of the RelatedItemsWidget or a customized version of the AjaxSelectWidget for related fields would be helpful.

BTW, where is the template for the RelatedItemsWidget? It seems to be not in p.a.z3cform.