How can I make an even ground for a garden nursery, without using concrete?
4" of packed gravel (tamped every 2" with a plate vibrator) will meet all your requirements: cost, DIY and drainage.  In fact, unless you are sited over clay, no slope will be needed: rain will percolate through.

If you want a more formal surface, add an inch of sand and top with pavers or flagstone.

Use a mix of crushed 3/4 gravel with fines (particles all the way down to stone dust, 21AA in my area)

Plate vibrator: