If you have a Lidl nearby Pickle, they have bags of 8 tubers for 99p. (or they did last week, I'm assuming they have some left). Also have a look in 'search' about how to propogate from tubers now, before you plant them out. ;)
Found the item but don't know how to link it, so I've copied it.
Place the tuber in a tray of compost in a bit of heat.
The cuttings are the shoots of the tubers.
Cut them off when about 2" long, dip in rooting compound and then place in a gritty cutting compost in pots to grow on.
The tuber can still be planted out to grow on as normal.
The thread was titled 'cheap dahlias' if you want to read tho whole thing.