not look so far this year (need to though) but Argos had the best deals on blank cards with envelopes last year, & they are the smaller size that mean cheapest post price too
if you are worried about colours, you can't go far wrong if you stick with traditional Xmas colour combos, such as red, gold, green
we find things such as felt shapes to make scenes, highlighted with a bit of glitter worked well, & are pretty easy to get a little production line going if you are making a lot
for example, black felt circle for a Xmas pudding, topped off with a white semi circle, cut ragged at the lower end, to make the "runny brandy sauce" & a dab along the edge of the white, with a bit of PVA, dip in silver glitter & & add a few green leaf shaped sequins & red sequins, at the top for holly
or a long thing green triangle Xmas treem red felt square for its pot, & decorate it with multi colour sequins & some diagonale PVA stripes dipped in glitter to give a tinsel effect
lots more you can do with fairly simple shapes, Angels, prettily wrapped presents etc etc ......& you'll find buying your felt from a roll, (most places will let you buy 20 or 25cm) works out much cheaper than squares
Also use old ice cream tubs, or similar & putt your glitter into that, & DIP you card rather than sprinkle - less mess & less waste
good luck