I like Brent and Becky's Bulbs. They run two seasons, one for cool-season bulbs planted in fall (alliums are in this group) and warm-season summer bulbs. Late in each season, they have a sale of whatever is left.

That said, you may find these bulbs at a good price in the fall in retail stores.

I would grow these in groups of the same type so you get an orderly stand. Mixing them won't be as pleasing to the eye.

There are also many kinds of allium. I have mostly drumstick allium (1" across) and Christophii allium (2-3" across). In the fall you can find them sold in bags along with tulips, daffodils, etc. They are closer to 8 for $5. These are usually the purple globe alliums that bloom 2" across. The larger the bloom, the more costly the bulb. I love the giant ones, but I'm like you. I just can't pay that much. I also save seeds from them and trade the seeds. I know it will take several years to get a bloom, but that way I have a chance and other people have a chance, too, without spending every nickel we've got! In my area the allium sells fast in these larger stores. I have to get it quick or I've missed my chance. Maybe in large cities or bigger places they go on sale if there are some left over, but not here.
Heidi