seat post bolts won't fasten + spares?

Most likely they are standard bolts having a metric thread. To be sure, use caliper to measure the thread width (you measure the major diameter from a "hill" to a "hill", not the minor diameter from a "valley" to a "valley") and thread pitch (pick 10 threads, measure the distance between these, divide by 10).

If the thread matches the coarse threads at https://en.wikipedia.org/wiki/ISO_metric_screw_thread (for example if the width is 5mm and pitch is 0.8mm), you will very easily find a replacement. If the thread is a fine thread, finding a replacement will be slightly harder but not impossible.

It's also possible it's an imperial thread measured in inches: https://en.wikipedia.org/wiki/Unified_Thread_Standard

Also you need to be sure to replace the bolt with one having equal strength. If there's a marking such as 8.8 or 12.9 in the old bolt, be sure to select at least this strength. If the seller of a bolt doesn't specifically state the strength grade, most likely the bolt is junk -- don't buy such a bolt. If unsure, pick preferably 12.9 or at least 8.8.

You also need to select stainless vs ordinary steel. Stainless bolts usually can't be made to the highest grades, so ordinary steel bolts have a higher tensile strength and thus you can find bolt grades such as 12.9. However, in environments such as riding in salted roads without fenders where the bolts are subject to corrosion, stainless may be preferable.

Lastly, don't forget to select the correct length (measure the length of the bolt and pick a similar length), and don't forget to grease the bolt threads when installing the new bolt.