Argos play sand comes in 15Kg bags and cost £2.49 each, Check the bags tho when you pick them up as many of them have little puncture holes in them and may find by the time you get home there is not much sand left.
If you hold the bag in the center it will fold over on itself like a towel would over your arm which can make it easier to carry.
EDIT - Noticed you have a larger tank too, is it not possible to use some of the filter media from that bigger tank and put it in the new filter for the smaller tank, (you will need to replenish the larger filters media that has been "Donated" still.
Then do a 25% water change on the larger tank but instead of pouring the water down the drain put that in the smaller tank and then top both tanks up ?? that way you have instantly cycled new tank, can add fish as soon as you have the heater and wont need to get amonia nor wat to do a fishless cycle.