https://www.ebay.co.uk/itm/193879991087 ... SwFxlgHWJG
HDMI ADAPTER
https://www.ebay.co.uk/itm/113766801799 ... SwoCFiD4rw
VGA BOX WITH COMPOSITE AND S VIDEO
https://www.ebay.co.uk/itm/192897388692 ... xydlFSvCln
I have used and have all three of these and all do the job 480P I currently use the 3rd Link VGA Box Switch between VGA and Composite for Bangai O and other non VGA Capable Games which is very few since most can be Patched.
COMPOSITE And S-VIDEO Cables are also available but the VGA Box kind of defeats that since you could use any Composite and S-Video with the VGA BOX and of course VGA.
RF CABLE. I used to use this back in the day for so long Dreamcast Launch I dont miss it but miss 90s

https://www.ebay.co.uk/itm/224854097769 ... SwNTdiF1-M
RGB SCART
https://www.ebay.co.uk/itm/233188917341 ... SwZZlbmtb6
COMPONENT CABLE (These are a bit scarce and hard to find)
https://www.ebay.co.uk/itm/154522933166 ... SwJ1lg5r5N
ALTERNATIVELY You could make your own if Component is what your after get and XBOX/PS2 Component and mod Connector with Dreamcast Connector or whatever Cable your after?
@KelSolaar