Well being in UK, you're closer to Ultimarc then the US and Canadian bunch of us.
www.ultimarc.com under the joysticks section, he has a whole list of joysticks and buttons. If you want official joysticks that almost all american arcades use, then go over to
www.happcontrols.com I'm pretty sure they have a camp setup over there in the UK. If not the shipping probably wouldn't be too bad from the US.
Ok, as for the interface, make sure you DO NOT hack a dreamcast pad to your set of interfaces, because otherwise the DC's gnd will draw +5v from all the other pads and none of them will work unless your dreamcast is on and plugged in. So what you want to do is first pick up a 5 dollar offbrand psx controller, I highly suggest the Pelican digital pads, they're very cheap and easy to replace if broken in the process of soldering. Then go over to ebay, and look up psx to dreamcast or psx to dc and find a total control 1 convertor. Or you can buy one here, this is the EXACT one I have on my dreamcast, and it works flawlessly,
http://www.ps2modchip.com/search.php?page=dc7 Ok, for the neo geo hack, if you're computer is too slow to run NeoRage0.8c *
www.anigames.it * or you are hardcore like my bro, then here's a layout for hacking yourself the controller though a joystick port.
http://www.gamesx.com/controldata/superneo.htm And for the pc part, you either can hack a sidewinder for 5 bucks,
http://shop.store.yahoo.com/surpluscomputers/micsidjoy.html or go on ebay and look up psx to usb to get one of those convertors.
-Good Luck
-CthulhuLuke