Aliexpress - the Chinese on-line seller site.
As far as I know - and I did a lot of research - there are very few problems using this (at least no more than any other on-line seller site). That said, I'm reluctant to recommend a particular seller just in case... There is a thread on Reddit about Lepin and the sellers that offer the products. It reviews sellers on a monthly basis and gives some advice. The August review is here:
Aliexpress store ratings & discussion + general information - August 2017 • r/lepin. Basically, if a seller has a high rating (three diamonds) then you're going to be OK. Prices seem to be more or less the same across the board so it's not really worth trying to save a few quid by going with a new seller: I paid 160 Euro with DHL included.
Problems seems to be with certain sellers giving a DHL tracking no. that is just a "holding number" (i.e. it doesn't work). But even then the package is just delayed - it does arrive. Mine was delayed due to a mix up with (genuine) numbers and took two weeks. No VAT or duties to customs duties pay as the seller declared a value of 20 Euro (considering the size of the package I'm surprised it got through, but there you go...). Other problems are with missing pieces but this seems to be rare. I actually ended up with more than 300 extra pieces with my MF! And I just took delivery of the Red Five X-Wing (50 Euro incl. DHL) and had precisely the correct number of pieces (including the handful extra pieces that the original version had). Apparently if there are missing pieces it tends to be a missing bag. Lepin sort their pieces into individual bags (rather than in order of build). The advantage of this is that it's easy to check alongside an official parts list. The disadvantage is that you need to open the 100 bags and set out all the pieces (hence me taking over the spare room).
If you look around the Net you will find some comments re. Lepin quality along the lines of "won't let it into my house - nor will my eight year old son". However, back on planet Earth it seems that there is no difference between it and Lego. With the MF apparently the blue power cells at the back are a slightly different shade of blue and the radar disk is slightly smaller. And that's it. In fact Lepin seem to have improved the "clutch" power of the technic pieces that attach the landing pads. The minifigs are not the same quality though. From the few other Lego sets I have I could not tell a difference.
Finally, the UCS MF is a beast of a thing; the table mine is sitting on is 60x70cm. It was a fantastic thing to put together - although some pieces seemed redundant, and a lot of the 5200 piece count is made up with tiny little bits that sit here and there (valves and things I suppose). The top panels seem to sit on the model rather than connect which is a little disappointing. However, it's an exact copy of the genuine article so the fault was there also.
One last thing, it's worth downloading a copy of the genuine instruction booklet; there are a few differences (these can be found on-line) but I only spotted one or two that made a difference. Otherwise, the Lepin booklet is the same except it puts two or three steps on each page (A3 size) rather than one step per page (also it very helpfully highlights the pieces in their place on the main model) - nice when you're putting a small grey piece onto a larger grey piece!
Phew! Long post for a simple question but I thought you might like the extra info.