You can buy them online from many sources. Quality bareroot 5 ft trees are shipped dormant in March from many reputable nurseries for a reasonable price. I happen to buy my trees from Boyers from Biglerville.
A quick look at QDMA forum under will provide additional sources
Word of advice. Buy grafted trees on rootstock EMLA - 111, 118 or even M-7. 111 will be just a couple ft shorter than standard apple. 118 is about full size and m-7 will be shorter than 111 but bear sooner. Also look for disease resistant varieties such as Liberty, Enterprise, Freedom, Goldrush.
Here's a list from ACN nursery(one of the most preferred nurseries) for disease resistant varieties along with their ripe dates.