This Instructable is about how to make your own. The basic ingredients cost me about 10-15 pounds, but I've got 9 or 10 beaters out of them so far, so you can see the cost saving. Of course it gets cheaper if you can recycle old wooden broom handles, stuffing from broken stuffed toys etc. If your beater has broken off right near the head, you might even be able to reuse the shaft to make a new one.
Step 1: Get your stuff together
Round wooden stick/ dowelling, 2-3 cm (0.8 - 1.2 ins) diameter.
Broom handles work well and I've also used a recycled chair leg. You could also use aluminium tubing
Self-adhesive rubber foam sealing strip
You can find this in any DIY shop, being sold to seal window or door frames, round the edges of baths etc. Go for something between 1-3 cm wide, and with a decent layer of foam. Boat stores/ ship chandlers etc. also sell similar stuff as weather seal for boats
Synthetic or cotton padding/ stuffing (I think it's known as batting in the US)
You're looking for the wadding you use to stuff quilts or coats. If you can't get that, toy stuffing works as well. You can often get a large amount very cheaply
2 or 3 round latex balloons
Elastic/ rubber band
Thick towelling sports socks or hiking socks work better than thin cotton ones.
Duct/ gaffer tape
Electrical insulating tape