Secondary UD and RHH/RHW/USE are not the same. They do have a similar insulation but the aluminum conductors are different materials. The UD uses a 1350 series that is not allowed inside the building or dwelling per the NEC. The USE has a 8000 series that is approved for inside.
The insulation is not approved for indoors either unless it is of a RHH/RHW type. What you are talking about is USE only with no other rating.
What is sold at big box stores is most likely acceptable for use in buildings, they tend to not sell the other types because they will get a lot of complaints from the people that get it turned down by inspectors. If there are locations selling the stuff that is not dual rated they probably do not have inspectors that enforce this rule in the area.
We used to have a similar situation around here years ago with supply houses not stocking 6 and 8 in green and white. People always marked them just like they did larger conductors. When inspectors started to make people change them where needed they eventually started stocking them.
Wire and cable companies can make just about anything, suppliers usually only carry what sells the most. They can special order but you usually need to buy larger quantities and not shorter cut lengths.
