I've looked everywhere for concrete blocks, available for purchase at various companies but you need to be a business to buy from them. I haven't been able to find them online or at hardware stores. I'm strongly considering making them myself. Does anyone have any tips on how I should make them or alternatively where they can be bought? I would prefer to have tripods with rounded corners, though it might be difficult to make myself...
Sketch how they should look
Just found spacers, searched for arch distance and then found plastic spacers with a small bearing surface. These should work even though they are plastic and not concrete. I will have a visible concrete surface and do not want the spacers to be too visible.
Curious Is it then to keep the reinforcement in place, I made my own with wire in the mold was small hutteglas (bowl) in plastic
Yep to the reinforcement, I was also considering making my own in small glass. The problem is that I don't want them to be visible when I remove the form. I'll have to try with the arch spacers and hope it turns out well. 
On the underside as I understand it, otherwise they won't be visible. The smallest I know are "plastis," which have four legs against the mold and four legs upwards to clamp down the iron approximately 3 cm from the mold bottom. They will be visible on the underside as four small plastic blobs. Even if you make your own, they will also appear as a round blob. I don't know if you can use the other ones with a large foot (available in stores, red, green, yellow) for different heights. Lift and shake the irons a bit when you pour, which may be difficult if you have a concrete truck on site, but the guys are usually good at it. This way, concrete seeps between them and the mold, just make sure they stand straight.