Back in the days of Rhythms and Copper Mountain gear, Netopia had the D
series routers which were actually xDSL to DSU units.
Yes, I am very familiar with them:
http://ifctfvax.Harhan.ORG/OpenSDSL/existing_cpe/netopia/dsu.html
As that page explains, they are only pseudo-DSUs though. I much much
prefer a true bit-transparent DSU:
http://ifctfvax.Harhan.ORG/OpenSDSL/existing_cpe/xsb2000.html
I have also designed and built an SDSL to EIA-530 DSU of my very own:
http://ifctfvax.Harhan.ORG/OpenSDSL/OSDCU/
On the latter I have the hardware (the page above has a photo of the
board), but the operational software (or firmware if you will) remains
to be finished.
Though, I'm not sure they'd work these days,
Only in the very limited geographic footpring of what used to be DSL.net
- they are the last remaining semi-major operator of CM DSLAMs:
http://ifctfvax.Harhan.ORG/OpenSDSL/megapath.html
My big goal is to make my own DSU which I have just mentioned function
as a Layer 2 converter (FRF.8 and friends) from Nokia SDSL/ATM served by
Covad to HDLC.
nor do I think
they came in ADSL models either.
I don't think anyone have *ever* used V.35 & friends with ADSL -
probably because those who would want V.35 (i.e., people like me) would
find ADSL morally offensive. 
MS