There are quite a few online places where you can get the D-Link DSL-2750U from, which is quite a good ADSL modem. So far I've seen 1 complaint about it where it gave serious issues and then a replacement with the same model fixed it for that user.
The other option would be the TP-Link W8960N, which is also in the same price range as the D-Link DSL-2750U, but that you'll most likely only find at Uniterm Direct (cheapest when you go via Bid-or-Buy from Uniterm Direct, since their own website lists the products as Excl VAT).
If your signal is still poor, then play around with the antenna orientations. The antennas should be perpendicular to the area that the signal should be broadcasted. So if you want the signal to propagate horizontally, they should point straight up. Sometimes it helps by tilting them slightly.
If adjusting the antenna's or router placement didn't help enough, then having another WiFi AP/router at the other end with external antennas might be your only option besides EoP (Ethernet over Power) or LAN cable options.