The short answer
No. Possessing a controlled substance is a federal offense unless federal drug law itself authorizes it (21 U.S.C. § 844. Penalties for simple possession), and marijuana is a controlled substance under those schedules (21 U.S.C. § 812. Schedules of controlled substances). Crossing a state line does not change that. The answer is the same where the state you leave and the state you arrive in both license stores. Buy in the state you are in, from a store that state licensed.
The detail
You commit the offense by possessing knowingly or intentionally, and a first offense can bring a prison term of up to one year, a fine of at least $1,000, or both (21 U.S.C. § 844. Penalties for simple possession).
Section 844 sets out three exceptions. One is obtaining the substance directly from a practitioner who is acting in the course of professional practice. Another is obtaining it under a valid prescription or order from that practitioner. The third is any other authorization in the Controlled Substances Act itself, or in the import and export law beside it (21 U.S.C. § 844. Penalties for simple possession).
Handing any of the cannabis to another person falls under a different section. Section 841 makes it unlawful to distribute a controlled substance, or to possess one with the intent to distribute it, except as that subchapter authorizes. For less than 50 kilograms of marijuana it sets a prison term of up to five years for a first offense, a fine, or both. Where the amount is small and nobody pays for it, the penalty is the one section 844 sets rather than the one above (21 U.S.C. § 841. Prohibited acts A).
Flying does not change the offense either. Section 844 reaches any person who possesses a controlled substance, and it makes no exception for an airport, for an aircraft, or for the state you took off in (21 U.S.C. § 844. Penalties for simple possession).
What varies by state
What you can buy when you arrive is decided by the state you arrive in. A recreational or medical dispensary is licensed in 43 states and districts. This site's page for each of them states that state's age rule, its limit on what you may buy and hold at a time, and its answer on whether you need a card. Each carries the statute or regulation beside it, or says so where that state's own text could not be read.
- Before you leave home, open cannabis law, state by state and pick the state you are traveling to.
- Read what that state licenses, the age it sets and the amount it allows.
- Follow the link from that page to the state's own store list, where this site holds one, and pick a licensed location before you go.
- Read what that state says about possession as well as about buying, because its limit applies to what you hold there after you buy.
This site lists no recreational or medical dispensary in the other 8 states. In 5 of them the only counter you can lawfully buy cannabis products from sells hemp-derived ones: Kansas, South Carolina, Tennessee, Wisconsin and Wyoming. Another 2 license no cannabis shop of any kind: Idaho and North Carolina. What either state permits without a license is on its own page. This site cannot say for Indiana, which publishes its Code in a form this site could not read.
What to watch out for
The common mistake is buying more than you will use before you leave, because whatever is left over is in your possession on the way home. Buy for the days you are there.
An airport, an airline or a state agency can publish a policy of its own, and section 844 makes no exception for one. A card from a state's medical program authorizes you under that state's law. What section 844 asks is whether a practitioner's prescription or order, or federal drug law itself, stands behind the substance you hold (21 U.S.C. § 844. Penalties for simple possession).
Where to go next
What federal law says about weed sets out how federal law classifies marijuana and what it prohibits, with the citation beside each statement.
Cannabis law, state by state holds a page for every state and the District of Columbia, and one for federal law.
The store directory searches by city or by ZIP code. Each store page names the authority that licensed the location and the date the record was last checked, with the license number where the state publishes one.