A few handy items for Warlocks:
1. Bracers of Blast Barrier: 3,200gp price tag makes these available around level 5. Turn your Eldritch Blast (or any other damaging invocation) into a 10*10 wall 3/day with a duration based on spell level (if using your Blast for this, apply an Esccences). Not bad as it offers some battlefield control long before you gain access to any of those invocations (not counting Wall of Gloom).
2. Bracers of Entangling Blast: Less expensive at 2,000gp and more useful to boot. 3/day, you can alter any invocation to deal half damage, but entangles anyone damaged by it for 1d3 rounds (no save) During each round, they take 1 damage per spell level. This is best used when combined with a damage over time invocation (Wall of Perilous Flame actually ends up dealing more damage this way) as the entangling will re-trigger each time they take damage. In the meantime, you just gave them a -4 penalty to Dexterity, -2 penalty on attack rolls (total of -4 for Dex-based attacks), and halved their movement speed. Not bad, and the price is right, making this item available early on.
3. Gloves of Eldritch Adimixture: 2,500 gp to add fire, cold, or acid damage to your Eldritch Blast up to 3/day, depending on how you spend the charges. It's bonus damage, which can be handy.