I don't think that any company has bothered to make such a low-volume product. Have you done a Google search for "scsi to usb?" Here's a result: http://www.scsisource.com/scsi_to_ide_adapters/
, but they are expensive.
1) What _kind_ of SCSI, and what kind of connector? There are 50, 68, and 80-pin flavors. Low-voltage differential and single-ended. Internal connectors and external connectors. What's compatible with one may not be compatible with another.
2) Find somebody you know with a machine with a SCSI controller and use that machine for an hour. I've got three SCSI controllers in my house, with the Ultra320 actually in use, but you probably don't live near me.
3) Pick up either a SCSI controller or some random device picked out from the Google search above. Before choosing one, be absolutely sure that you know what kind of SCSI you need (see 1).
Unless you know someone with a SCSI controller that he/she could lend you the use of, you are in for an expense.