As to the best place to get a mini lemon...I'd suggest looking for threads posted by people who've bought from different suppliers. Plant health, and nursery culture, combined with top notch packing and shipping would be what I'd look for in commentary.Google for suppliers, and then google them individually to look for commentary, independent of their websites
I'd also suggest waiting until well after all threat of frost is gone before ordering one. Don't want a cold sensitive citrus sapling sitting overnight in a semi truck somewhere in freezing temperatures.
A couple things to be mindful of.. even a "mini" citrus cultivar will try to get 8-10 feet tall, so you'll have to keep it reduced above, and probably below ground to maintain the size that you want. Also, most lemon and lime cultivars have thorns, so if you've got little ones wandering around, that may be a consideration.