You need to practice with whatever you buy. You say ammo is hard to come by. Have you considered buying an additional airgun for practice? I don't know if your laws consider these "firearms" and thus regulate them.

Buy yourself an inexpensive Crosman 1377 or 2240 air pistol to practice trigger control, marksmanship, etc. For practicing marksmanship, an "airgun" would be better than an "air soft gun". Airsoft are lower velocity and power and use different ammo and are not very accurate. I consider them more as toys - but fun toys! In the U.S you can buy a new 1377 (pump action) for $49 and a 2240 (CO2 cartridge) for only a few dollars more than the 1377. The 1377 will be far cheaper in the long run since you don't have to buy CO2 cartridges for it. You may want to do a trigger job on these guns, but that only takes you an hour or two and about $4 worth of parts from a hardware store. Very easy, and a vast improvement. You can buy airgun ammo at about $8 for 500 great quality pellets, $4 for 500 decent quality, or $2.50 for 500 lower end pellets. Unless you're into high accuracy and target shooting, you can use the cheaper pellets for practicing sight picture, trigger control, etc. A decent quality air rifle (as opposed to air pistol) would run you in the neighborhood of $100 - $150 if you shop around.