Yes, I also found it hard to find attractive obelisks that were tall enough for my 4 plants in large containers. After I installed the ones I settled on, I saw some decent ones at my local nursery waaaay in the back. I thought I knew their inventory but these were tucked away and they looked like they had been there awhile, spiderwebs and such. Guess there's not much demand in my area. Ask, they might have them in the back.... These are the ones I bought last year but I wouldn't recommend them for clematis since they are narrow at the top and the vines seem constricted, also they aren't holding-up. http://www.gardeners.com/buy/expandable-willow-trellises/37-440RS.html#start=4

Clematis come in a range of sizes - how large is yours?

I have found 6' trellises at my local nursery (but made by a California craftsperson) and at Ocean State Job Lots. I've also made some taller ones using 8'-10' long pieces of 3/8" iron rod (rebar also works) fastened together at the top (wire or a hose clamp work well) and with a spiral of wire to give the clematis something else to climb. For my largest clematis, I plant them near a shrub they can scramble up into. If you aren't handy and can't find anything locally, you might find a welder in your area willing to make some for you.

Build your own! Tuteurs - a wooden obelisk - are very easy to construct with basic woodworking skills and you can design them to a size that works best for you.

Tuteur plans.

I am done buying obelisks and am going to make these. I could make 3 or more of these for the price of one on amazon.

http://forums.gardenweb.com/discussions/1582417/rebar-arbors-tripods?n=8