You can try the various DIY and Hardware shops, listed under “Shopping” for small quantities of the more common acids and chemicals - with I Nova my personal preference.
Alternatively, “Ramires & Ramires” is a good Builders’ Merchant. Their store is 1 km along EN 125, heading towards Portimão, on the right, just past the BP garage and a small garden centre. You may have to buy larger quantities, as it there business is aimed at professional builders, but by using our VHF Net or Bulletin Board, you may be able to share it out among your fellow yachtsmen.