Any of those "big three" online stores will have what you need. I like Red Star wine yeasts- they are about $.89 and come in many different kinds. I use cotes de blanc, montrachet, premier cuvee and champagne yeast the most. For Lalvin yeasts, I use the 71B-1122 or EC1118 the most. One package is good for up to 6 gallons, so it's pretty economical.
For dry beer yeasts, I use s-05 and nottingham the most. They are neutral ale yeasts.