Codetel would be your best choice. But prepare to pay from US$ 50 a month for a slow connection. I pay RD$ 4200 monthly for 1.5 Mb/s - 768 kB/s (down/up stream) - that includes all taxes.
Cable TV companies also offer internet via the cable. But according to friends who use it the service in unreliable.
Also, there are many open wifi spots available in Sosua. If you use powerful WiFi adapter you can always get at least one hotspot in your area.
