The last external plenum I bought came from xtremeairguns in South Africa. pricing was fair and shipping didn't take too long. IMO, my preferred route to go on a .25 P15 pellet shooter is not to add hammer weight and/or stiffer springs.....but to maximize hammer stroke and make the valve easier to open....Hammer stroke----You may find stem protrusion to be a bit excessive and have room to trim off approximately .5-1mm. With the P15's short hammer stroke and light hammer, every little bit of travel helps. Be careful though, if you go with too little stem protrusion you will bounce the hammer off the valve and thus make very little power.Make valve easier to open---The OEM poppets I've seen are significantly oversized, making the valve harder to open than it has to be for the P15's relatively small throat diameter. Reducing poppet diameter to just over the valve throat diameter (while leaving sufficient sealing margin) will help. I've done this with the OEM poppet but ultimately started making my own poppets out of peek. Combining a peek poppet, sized just barely over throat diameter, and a 2mm stem will allow you to make the valve easier to open AND increase flow. With this combination you will find the OEM hammer spring to have more than sufficient preload even at reg settings upwards of 150 bar.