Go to Ace Hardware. They have an awsome selection of metric. Lowe's and Home Depot are abysmal. Also make sure you get some locking washers of somekind. The exhaust in particular can be a pain because the heat and the flex will work look regular bolts. Thats why the spring is used on those types of bolts.
I had my catalytic convertor actually fall off the header of my Lotus one time because of this. And it was a total bitch because it was during rush hour traffic so I was in stop and go trying to keep and engine alive that was loud as hell and couldn't read the air fuel ratio (since the downpipe was disconnected).
Also try Autozone and other local auto suppliers for actual exhaust bolt kits.
Bolt is an M10 with a 1.25 thread pitch, repeat 1.25. It goes into a captive M10 x 1.25 nut on the exhaust flange.
Is 60mm long overall and the collar you see is so that the bolt can seat against the top flange and so lock without fully compressing the spring. Not a good idea to fully compress the spring to lock the bolt as this stops movement in the joint, which is needed.
So if you cant get a replacement unit then you can put a tubular spacer over an M10 x 1.25 hex head bolt shank to space it to the Exhaust maniflold plate, with a nice thick 23 to 25mm dia M10 washer to retain the spring.
If you can only get a longer bolt then check that the shank is not too long or it will bind in the exhaust flange nut.
Best mild steel tubing for the bolt shank would be 15mm OD max , 11mm ID and cut to length of stock bolt collar.
I was having so much trouble because we stripped the threads on the bolt and the flange, so we tapped a new one and got new bolts, all is good now and the car gets lots of dirty looks because it's so loud - so it's great.