There are lots of independent suppliers.
autohausaz.com
penskeparts.com
partsgeek.com
fcpgroton.com
bmwmobiletradition-online.com
bimmerdoc.com
One thing to know about the aftermarket is that a lot of guys appear to use the same wholesale service. If you start seeing the same interface over and over, your supplier is using the same wholesaler and parts prices will vary by only a little.
Look around - prices can vary quite a bit.
Sometimes a vendor has a sale but only with luck will you actually need the part they're selling.
The cheapest vendor may not be the best supplier - you have to factor in price of shipping.
It's good to know the brand of part you are ordering. Most internet suppliers also have an 800 number you can call and verify brand. For instance a Brand X clutch would be a risky purchase at any price, over a F&S clutch.