What do you want the trees for? Fruit, nuts, shade, timber? How much work do you want to put into the trees (pruning, etc.)? The more dwarfed you go with trees the more work they need to maintain form, not blow over in strong winds, etc.
I'd establish some goals and a planting scheme before I just bought trees, as once they come in (bare-rooted) you have limited time to get them in ground. For that matter what kind of soil do you have? Is the drainage good? Any terrain features to work around? For that matter where do you live?
If you're wanting suggestions on good trees you've gotta give us more info. I can give you great suggestions just based on my area, but I doubt your climate is the same.
Anyway, Stark Brothers is where I got the bulk of my trees. Also purchased some from Burnt-Ridge nursery I couldn't find elsewhere. Raintreee Nursery is another solid option, groworganic.com. It's as easy as a Google search really, or for that matter go to Lowes/Home Depot and just buy some. They should stock stuff applicable to your area, just make sure you buy a pollinator if the tree needs one. I'd personally say wait til fall. A better selection will be available (you're kinda late to bat so to speak) and you won't have to worry about watering the trees as they establish, assuming you have to wet fall/winter.
Arbor Day Foundation has decent prices. Lawyer's Nursery has good pricing on bulk. England's Orchard and Cummins Orchard are some good sources for fruit and nut trees. Raintree is also good. Hidden Springs is a good one I have bookmarked for next season because they are out of a lot right now. Lastly, One Green World is good for permie type trees on the west coast. You can price shop with Stark Bros, who is a big operation but many small places can be closer and cheaper too.
Depends on variety and size requirements. I've ordered large trees from Starkbros.com and Southmeadows Fruit Garden. I've ordered tiny trees from coldstreamfarms.com and oikos tree crops. I've also ordered things on Ebay.