+1 on Fly Tyer's Dungeon for synthetics. Great selection and literally unbeatable prices.
I'm assuming you don't have a local brick-and-mortar fly tackle retailer, since you're asking about other sources. However, if you do have something local, check them out first. If they have what you need and provide good prices and service, I would say support them if you can. If, however, you are relegated to online sources...
For dry fly hackle I recommend Collins Hackle Farm (www.collinshacklefarm.com). You have to order by mail or phone, but the inconvenience (if you consider it such) is well worth it. Free saddle patch with every cape purchased, and I have found the quality to be very good. Mr. Collins is also an interesting guy to talk to.
For natural hair and fur in general I recommend Whitetail Fly Tieing Supplies (misspelling theirs, not mine; www.whitetailflytieing.com). I've yet to come across another retailer that can match the variety of his selection.
Specifically for deer belly hair for spinning/stacking/packing floating deer hair bass bugs, I recommend Pat Cohen's site www.rusperfly.com. His prices are reasonable, service is consistent and quality of the deer hair is consistently excellent.
For hooks, beads, dumbbell eyes, coneheads, mono eyes etc., I very highly recommend Wholesale Fly Company (www.wholesaleflycompany.com). I learned about them only recently, but since then have placed 3 orders and have found the quality of their goods, their prices and their service all to be outstanding.
For more of a "one-stop-shopping" experience, I would recommend Bear's Den (www.bearsden.com). Both Bear's Den and Wholesale Fly Co. both charge only the actual shipping cost, not a flat fee. This is a big selling point for me.
Good luck and welcome back to the....oops I almost said "fraternity" but it's not all guys anymore...welcome back!