Well, there are really several different issues involved. The first is actually obtaining a "compassionate use" approval from the FDA. This requires that the plastic surgeon be willing to invest his own (unpaid) time and effort to file for this special FDA approval ... time that could be spent putting 300cc implants into an extra housewife or two.
Second, a "compassionate use" approval does not equal an FDA device approval, which foreign made implants of any size will not have. Without an FDA device approval, no hospital or clinic will allow a medical device to be used on their premisis, as they are bound by FDA rules for medical facilities. Therefore, any plastic surgeon wishing to implant foreign made implants will also need to have invested beaucoup bucks into his own private surgical facility (which is not subject to the public medical facility FDA rules). This fact alone probably rules out 3 out of every 4 plastic surgeons.
Third, many plastic surgeons belong, or seek to belong, to the ASPRS and to become "board certified" - primarily as an advertising tool/higher fee justification for housewives. The ASPRS severely frowns on "experimental procedures" being done by its members, and might very well deny membership to a plastic surgeon who has done large foreign breast implants.
Lastly and perhaps most significantly is the unavoidable issue of medical malpractice insurance cost. The actual cost that an established plastic surgeon must pay for this insurance is based on past history of lawsuits, as well as records of surgeries performed. Since large implants inherently have a higher complication rate due to the greater degree of skin stretching, the greater distribution of existing vascular system etc. a plastic surgeon risks having to pay astronomical future insurance premiums if a large implant recipient with complications should officially raise a complaint or a lawsuit.
If you put all of these factors together you can begin to understand why very few US plastic surgeons are still willing to work with large foreign made implants - particularly for new patients. In fact, some of these factors have now become so negative and that at least one plastic surgeon who had been willing to work with large implants in the past has stopped doing them. It is getting to the point where even doctors who are big boob lovers themselves cannot afford to indulge their favorite surgical subject, because they can't afford the potential negative impact on their balance sheet if they wind up in court or wind up paying an extra 100 grand a year in malpractice insurance or if they have to charge every housewife $1000 less than the doctor down the block because they can't become "board certified".
Assuming that you have made it past the above obstacles and have located a doctor who is willing to work with big implants, you now have the issue of having the big implants custom made. Both of the well known companies who are willing to make big custom implants are located in France. So in the first place you have to communicate with them, inform them of what size implants your doctor thinks are proper, and what type (i.e. silicone, saline, adjustable, double bag etc.). Then you need to pay up front to have three custom implants made (gotta order your own spare). Custom made implants are not exchangeable or refundable.
3 or 4 months later your custom implants are ready - in France! You now have the issue of arranging for importing them into the USA, paying appropriate customs duty etc. and hoping that they actually arrive without damage and without breaking the sterile packaging. This task is much more complicated than it used to be due to increased US customs inspections after 9/11. When I ordered my first set of 3 custom made implants several years ago, one was found to be leaky on the operating table, and the second failed about 3 days after my surgery. After another 3 months of waiting and the cost of two more custom made implants and a second surgical fee, I finally had a "working set" and a spare (implant company is not responsible for damage done in shipment or customs inspection).
Lately, it seems that some girls are avoiding these problems by simply travelling to France to have the surgery done. There are no FDA implant approval or importation issues to deal with if a French plastic surgeon does the surgery in a French medical facility using French implants. The tradeoff is, of course, that the doctor is 4000 miles away should problems develop down the road.