If I remember my figures correctly, a single 1.5Mbps ADSL connection (downstream, at least) can carry the same amount of bandwidth as a T1. (A T1 consists of 24 64-bit channels, totalling 1.544 Mbps.) Get yourself a HUGE router for 36 PCs, and you'd have the equivalent of a T1.
However, if you need the same upstream bandwidth as downstream, a 1.5Mbps SDSL would be very expensive (if it's even available), and possibly not much less than an actual T1. (Maybe two 640kbps SDSL lines would do the trick for you. That would only run about $350.)