I've printed all four rifles from this set using my MPSMV2 (Monoprice Select Mini V2) printer. The best spring I can find in the USA is from Walmart: Mainstays plastic clothespins 72 pack for a few bucks. The metal springs are thicker than the standard wooden clothespins and are stronger. I'm using cut round toothpicks. I first tried the 2 piece low and high and glued the parts with CA. I find that the one piece prints fine without any modifications without the need to use glue.
Hobbyking 1kg standard PLA (China) PLA