I have the Swiss-designed Varrox unit, and it is excellent. Expensive, but durable, reliable and nicely made. You can buy one here:
https://www.thorne.co.uk/index.php?route=product/search&filter_name=varrox
You then need a power supply, and I use a lightweight jump starter, as one of my hives is a long way from civilization. 
Any 12V jump starter designed for SUVs, trucks etc, should be more than good enough. Mine will do about 5 hives before needing a recharge.
For the Oxalic Acid, I just use commercial wood bleach. In the US, this is 99% pure oxalic acid crystals, which is more than good enough. In the UK, B&Q or Homebase should have something equivalent. 