If you're going to spend nearly $150 USD on an adapter, one of the best is an Acard 50 pin SCSI to 2.5" SATA adapter. It fits wonderfully, plus SATA SSDs are really easy to find. Since full height drives fit in the SE/30, another good option if a spinning disk drive is fine is the ARS 2000FU which mounts under an IDE drive and provides a 50 pin SCSI connection:

http://www.acard.com/english/fb01-product.jsp?idno_no=26&prod_no=ARS-2000FU&type1_title=SCSIDE%20Bridge&type1_idno=2