In Chapter 14 after the anti gravity section (which is straight after that respawning bastard) you get to a room that you must unlock with a node, in it are 2 nodes and a ruby semiconductor. Pick those up and continue your journey and you come to a large save game room complete with an upgrade bench and store, save the game and once saved click on restart checkpoint, when it reloads head back into that node unlocked room and 70% - 80% of the time there are 2 nodes and a ruby semiconductor (which you can sell for 10000) there again. Pick em up and go to the save game room and save then checkpoint restart. I must've done it 50 times and bought every weapon and fully upgraded them all as well as stocking up on ammo and health, wear the suit that gives you 15% store bonus so you can get more items for your money. I only went to the store and upgrade bench after 4-5 visits to the node room to save time and again after another 4-5 visits of building up nodes and semiconductors.