Even weeks after its early April launch, the Apple iPad is still pretty scarce in U.S. retail stores.

As Apple prepares for its international launch, it still hasn’t succeeded in completely stocking the shelves of its stores, according to Piper Jaffray analyst Gene Munster. He checked in with 50 U.S. Apple stores and found the iPad to be sold out or in limited supply.

“74 percent of the stores we checked were completely sold out of all iPad models,” Munster said in a note to clients. “26 percent had some WiFi models in stock, and no stores had any 3G models available. We note that the stores have implemented a reservation system, by which customers can request to be placed on a wait list for future iPad shipments, notified upon arrival, and given 24 hours to pick up the iPad. One Apple store representative indicated that the in-store reservation system typically takes 4-7 days.”

Munster estimates that the shortage and the coming international launch next week means that Apple’s supply constraints will last through the end of the June 30 quarter. Apple said it sold its first million units in 28 days. While there are many tablet computer designs in the works to imitate Apple, so far Apple doesn’t have much competition. It’s lucky in that respect; users don’t really have many other choices as they await the restocking.

Demand is probably a lot stronger than Apple anticipated. But there could also be supply problems, as Apple-Samsung is the only supplier for the iPad’s A4 microprocessor, and because Apple chose a unique 9.7-inch touchscreen for the device. If any single component is in short supply, then the whole product itself will be in short supply.

Yesterday, RBC Capital Markets estimated Apple is selling more than 200,000 iPads a week, and that means Apple is now selling more iPads per week than Macs.