Perennial cool season mix - I mix ladino white clover, some sort of perennial red clover and chicory. I have added alfalfa in the past, but it isn't required or anything "magic" in my use either.
As for a source - it depends on the size of your plot and what you have access to. A local seed supplier/coop should be able to help IF you have need for a decent amount....this can save you some money. Keep in mind they tend to carry varieties aimed are for feeding livestock. It will work fine, but they are not the varieties specifically created for deer. IF you simply have 1/2 acre or so plot in mind - there is no shame or harm in planting a BOB (buck-on-bag) type mix from Biologic, Antler King, or the other glossy bagged providers out there. Just read the label (seed tag) on the bag to make sure you understand what you are buying. Many of these mixes are distributed over a large geographic area so they tend to include some seed that you may not be interested in. Lower quality mixes will even include grass seed and the like as "fillers" just to get something green to grow. I buy these fancy bag mixes when I am in a hurry or just need a small amount - it's just more convenient. I have used Biologic, Antler King and Evolved Harvest brands before....again read the tag and make sure your getting what you want/need.