Unfortunately, it seems that BioSpira is temporarily unavailable to most of us in the US.
The next best thing would be to see if you can get a "starter" from a healthy tank of a friend - a used sponge or ask if you can put some lava rock or ceramic pieces in a bag in their tank for a week or two and then transfer them to your tank.
When you change the water it is all a dilution. The test kits still can read high, for you are only diluting the ammonia down a percentage and the tests are really not that accurate - they give a range of the amount of ammonia, not a hard and fast digital number ......
Koko is also right - you could get your ammonia to zero by dilution if you did enough, but your fish are overwhelming your tank and keeping the numbers high.
Sounds like you are on the right track at limiting their intake (and output) and with luck, you can get this tank cycled and working in a short time.