We have ordered from Thompson and Morgan several times. Always had good success with their seeds. They send a nice germination guide booklet with your order so you know what to do with the seeds when they arrive. Two of our favorite catalogs are High Country Gardens and Select Seeds.
Select Seeds has both plants and seeds, many of which I have never seen anywhere else.( Lots of Sweet peas) They send a "bonus" seed pack with your order.
Lots of heirloom and hard to find stuff there. They also give suggestions about companion plantings for containers and such. I followed their lead on several of my container plantings and got rave reviews from our gardening friends.
High Country Gardens doesn't have seeds, only plants but they are AWESOME.
Talk about unusual and hard to find stuff. They are out in New Mexico so some of their stuff won't grow everywhere. They give suggestions about what works in which zone though. A lot of their stuff cannot handle the wet Midwestern winters but we have had some success with various penstemons and agastaches that no one in our area has. Several of which have been going strong for 4 seasons now. I can't wait for spring to arrive. We are buried in snow now and the wind chill tonite is predicted to be 30 below zero. These garden catalogs help us get through the cold winter months.