Any cheap way of getting Epsom salts and bicarbonate of soda in qunatity?

[Wombat140]

What the title said.  Instructions call for one cup of each, local pharmacy and supermarket (respectively) sell them in little 100g tubs which obviously isn't ideal.  Any more cost-effective way of gettint them I don't know about?  NB am in UK if relevant.

[Luna]

Costco & Amazon

[Wombat140]

Ta