I have shipped woods and woods shipped to me internationally. Purchase only dimensional lumber for your purpose- looks like you are buying a very small amount with a quantity of six pieces. Make sure their invoice states the common name as well as the botanical name and they fill out the custom forms properly and provide you with proper importing docs. Make sure that there are no bark attached to the wood, no bug hole/worm trails, pin holes, feces trail, punky/spalt discoloration, knots, ingrown bark, incursions, pitch pockets to bring it into the US via air transport- it will go through an X-ray when it reaches customs. Make sure you request for a fumigation certificate from your vendors if these are present. If you don’t and customs halts your parcel, it will be quarantined. They will ask you whether or not you want to pay a fumigation fee or have it destroyed.
Someone here has experience by slow boat if you want to go that route- but, I won’t recommend that for woods to be used for instruments because the longer it stays not climate controlled, the higher the chance of you receiving it with checks, cracks, deformation, etc.
As far as the dimensions that are 48” x 8” would not be the way that I would specify it to get the sets that you need. If you will build a dreadnought- modern size clears at 8” wide per panel x 2 (modern is 16”, traditional is 15-7/8” at the lower bout). You really need 8-1/2” to 9” widths for slack. If you are building an OM- an 8” width or 7-3/4” width will do (22-21” long). I would ask for separate lumber for the sides- 36” long and 5-1/2” to 6” wide. For back sets, you only need 22” long max. So the backs should be 22” plus minimum and 9-8.5” wide. IMHO.
Before you do any of the above, ask them how did they dry the wood- air dried, kiln dried, etc.. Ask them to give you a moisture content reading. You don’t want green or wood with high moisture content. You can be specific and ask them what grain cut orientation you require. Ask for material clear of knots, knot shadows and other natural defects.