I buy them on amazon, from Fabric hut and Accuquilt.. On sale they arent that bad..
My favorites are:
2x2
2.5x2.5
Brick (3.5x6.5)
Rags- ALL
Tumbler- large and small
Hexagon- 9 in
and ALL the designers ones that i refuse to cut by hand, like DWR...
I have most dies, some I use more than others and some I used once or not at all yet..
I suggest you buy mats, lots of them. .IMHO they are disposable.. I go thru them very quickly with lots of hours and hours of use..