POC VPD Elbow pads are quite simply the best mountain bike body armour elbow guards there are.
Featuring VPD (Visco Elastic Polymer Dough) they are soft and mouldable but harden on impact. We can attest they do indeed work having been using them for years and avoided elbow injuries in crashes that resulted in broken wrists!
Buy some when we have them because they are hard to get.