any local tri clubs? Mine does an open water swim clinic, and sometimes members will post when they plan to do an OWS to get others to join them.

Like you said, Practice!
An open water swim doesn't have to be 1500m straight out into the middle of the lake and back.
It could be back and forth across the buoys that define the deep water edge of the swimming area. That's usually 200m to 400m so that's enough open water to gain experience.

Look, the factors that make open water different from the pool aren't that many or difficult to find:

1) Deep enough that you can't touch bottom (or even see it)
2) absence of lane ropes on each side so you have to navigate.
3) Turbulence, wave action, the sort of motion you might get on race day. Motor boats provide some wake action, learn to cope with it.

Swimming laps back and forth across the line of buoys is enough for what you want and it is safe enough not to need swim buddies.

Don't think about it, just do it.

PoC

You could do the aquathons and not worry about being slow... It will be good practice, if you are really off the back like you think you will be, then you won't have to deal with a lot of the thrashing that goes on and use it as an opportunity to safely practice your open water swimming. You could just do the swim and forget about the run, also if you are really that far behind. I've thought about going up to Madison to do one of those casual aquathons, but I haven't made it to one yet. Don't worry about getting your butt kicked. There is ALWAYS someone out there that can really kick your butt. Usually a lot of someones. You will get faster with the practice and it might be a way to make friends with others who can swim with you in the future.

http://www.hhmultisport.org/index.php?title=Home

A good tri-club in the Madison area.

I'm taking a second, closer, look at this. I hope you don't mind.

Okay, you have ruled out your local lake. You must find some open water to practice in. According to others, who know better than me, there are other lakes in your area. Go pick one out. Take a picnic lunch and make a day of it.
Or, going on vacation? There are some really nice, shallow, warm, open water areas to swim in in the Bahamas, or Ko Phi Phi maybe (below)

The second point, as Catwood noted above, is about attitude. Check yours.
If you seriously want to get better and eventually be competitive, decide that you don't care if you get beaten by a wide margin while you are still a novice. Even Zach had a bad day at Kona and finished way down amongst the age-groupers. - and we loved him for not dropping out! It happens. Get over it.

secondly, if you are unable to cope with being bad until you learn to be better, you will never be happy in this sport, spare yourself the misery, take up golf.

If triathlon was easy, everybody would do it.

PoC

Like POC was saying, just swimming in the deep end of a local swim area will give you 200m to 400m and that's definitely all you really need. I think the biggest thing for me during OW swims was going in a straight line, sighting, and getting used to not being able to see directly in front of your face. The lake I swim in is clean, but it's shallow so there's a lot of bottom sediment that's stirred up...it's weird not being able to see your stroke, like your arms in front of you and it take some getting used to for sure.