I have most of the seeds I'll use this year. I get a lot of them from the big box stores, or the "dollar" stores(love those 20 cent packs :grin: ).
Since they have gotten so espensive, I also try to grab last year's if they are selling them out on clearence at a liquidator or something,over the winter. I also like hitting local nursery, hardware, ag and feed stores for bulk bags and just to see what they have.
I've found over the years, that for most vegetables, seed that is a year or two old, germinates fine. Some, like sweet corn, benefit from being fresher.
As far as mail order, I've had good luck with Johnny's, over in Maine, especially for northern varieties, although they sell stuff for all climes. I usually look to them, or another catalog house, for something specific, rare or unusual I want that I cant find locally.
YMMV