I think the hubs are different enough to not be interchangeable. And as Amateb8 says, the thread pitch is different. Best bet is ebay. To find the required length do the following:
1.) Put one of your standard bolts through a standard wheel and measure the exposed length on the reverse of the wheel.
2.) Put that same bolt through your Alfasud wheel and do the same, note the difference (if any).
3.) Measure the shank length of the standard bolt
The new bolts need to be shank +/- difference + 15mm (for your spacers).

For example, if the shank is 30mm currently, exposed 12mm on standard wheels but only 8mm on the Alfasuds you need 30mm+4mm+15mm = 49mm.