I usually just rent it from minnow booster or blocktrades at whatever the going rate is.
minnow booster or delegation hub let you name what you want to rent for. going rate right now is about .5 or .6 steem per 100 steem rented where as blocktrades just rents it in 13 week blocks so for 1000 sp it cost you about 45 to 48 sbd. I just kinda float around and try to get the best deal I can on it.