Might I suggest, writing each of the alumni members of your chapter (really do not know how NPC works) and letting them know that you value an undergrad/alumni relationship FIRST. Thank them for their part as a member of your chpater, etc.
Then maybe if some are still in the area of your school and ARE INVOLVED with an alumni chapter, ask them if your undergrad chapter and their alumni chapter could get together for a service project or two (I assume that the alumni has the same programmatic thrusts as undergrad), have a tea or social that is intimate and from that broach the idea of them providing an advisor for your chapter or maybe a team of advisors. Sometimes being the sole advisor can be a bit overwhelming.
Good luck.