Look at the Predator range. The unique thing bout them is that they "shred" the muck as it is sucked in. It makes more room in the hopper and what you empty is already broken down.
They are not cheap, but then thats where ebay comes in.
I may be selling mine, due to change of circumstance...
We have had a Terra Vac for 3 years and it's amazing!
My sister does it all and she poo vacuums 12 horses once per week and does it in about 3 hours! If we did it by hand it would take about that everyday. Also it def improves the pasture as it picks up all the rubbish. Very very glad we got one! Ours also turns it to mulch and all you have to do is steer the quad...........no hand operated tubes!
Our local tactor and quad bike mechanics said they are the best on the market.