In the UK you can get it from Spices of India, http://www.spicesofindia.co.uk/acatalog/Jalpur-Papad-Flour-Urid-Dal-Flour.html#aRFG047 who do very fast mail order
In the USA you can get it from http://www.indianblend.com/site/664954/product/FL-13. I have never ordered from them so cannot report on their service, but it looks pretty straightforward to order from them,
Urid (or urad) lentil flour is made from a small pale cream coloured lentil ( it has a dark skin which is taken off first). It is used to make pappadums, idli, dhosa and other southern Indian pancakes and buns. It holds baked goods together much better than soya flour or chick pea flour, and doesn't leave any noticeable flavour or extra colour in the finished goods I make plain shortbread biscuits/cookies and vanilla cakes using it and people don't realise they are made without wheat.
I grind my own from the whole lentil because it amuses me and I have a beautiful Swiss stone-grinder flour mill. These packs of flour will make it easy to have good gluten free food that is suitable for anyone unless they have specific problems with intolerance to this lentil.
If you have any questions I am happy to try to help.