I just replaced my "sink wastes" (The fitting in the bottom the the sink) -

yea I did not know they were call that till I went looking for them.
The marine stores around here only had the sink wastes in 1" barb fitting. The RV place sink wastes were PVC glue in type or compresson like house hold type you fine at Lowes.
The only place I found Sink Wastes that reduced to 3/4" barb fittings was made by Forespar. I had to order them direct.
http://www.forespar.com/...alog/marelonCatalog.pdf
Check on catalog page 14 or PDF page 16 for the sink wastes.

Note If you have to replace one replace both. One of my orgional sink wastes fell apart. I checked the other one and it came apart in my hand.
"The is no dumb-ass vaccine" - Jimmy Buffet