The internet is not always the cheapest way.
For the best thermal properties (Tog ratings),you'll struggle to beat natural felt, we predominantly sell Axminster Axfelt65 which is a crumb rubber/natural felt combo. Natural Felt is a superior product to manmade felt, it's significantly better than puFoam and natural rubber waffle style underlays forcarpet wear, durability, appearence retention and support, you pays your money and takes your choice.
Personaly I would price up with the fitter, before making a decision, it's not uncommon for fitters to supply and fit at the underlay price or charge extra for fitting if you supply your own. Buying underlay online can be a minefield and you don't know what you are getting, quality and batches do vary, it would be safer if you have an issue to source the products from the same source.
Everyone needs to make a living, pricing every single penny out of a job does'nt give those supplying labour etc much of a living, have a heart and let the people who care about putting the product on your floors the opportunity to make a bit of a living/profit, they have mortgages to pay and families to feed too.