You could try magnesium citrate for constipation. I have heard it is the best form of magnesium for that (I get IBS -D so haven't tried it!!)
A pharmacy might have some in stock? Or get a good one with a reputable brand name from Amazon with next day delivery?
But don't start with a massive dose, or that would end up like a colonoscopy prep! Try a small amount and build up to what suits you, without giving you diarrhea.
Ground Flax seed might also help. You can probably easily buy Flax seed in a health food shop. One thing though...rather than buying a bag of ready-ground seeds, it's always better to buy the whole seeds, keep them in a sealed bag or container in the fridge, and grind them yourself (using a coffee grinder set to fine ground?)
The reason for that is Flax seeds can oxidise quickly and turn not so fresh or even rancid, and they do that faster when already ground, even if sold in a sealed bag. They are more susceptible to that than many other seeds.
But if you haven't got a grinder, never mind, just get a sealed pack of the ground variety from a health food shop.
I did taste some once at a friend's, and she bought them ready packed, and they did not taste rancid at all.