It's a game of patience over here in the UK. I started out desperate to just see a yew stave, and now a year or so later I've got enough for plenty of bows. That's not to sound cocky, it's just the way it goes. Make contacts with tree surgeons, landscape gardeners, churches, parks and planning people, leave them a nice business card with your name and number and what you do, and you'll soon find people dropping off logs and staves throughout the year as they cut it.
One thing I would say is avoid ash for full compass warbows. You'll only get disappointed by it! You can't make ash into a classic warbow shape, as it has to be heavily trapped and feature a very flat belly to withstand high poundage which won't feel right if you're used to holding warbows. Elm is great, Del just finished a stunning elm warbow.
Make sure you don't try and pay for a lovely piece of yew for your first bow. It will go wrong, come in greatly underweight and really frustrate you!
All that being said, keep eyes peeled on eBay. I used to get a lot of fairly good quality yew from sellers who were auctioning wood turning blanks and pens made from yew. Inevitability they had large chunks and logs they hadn't got round to chopping up, and didn't realise the real value, so I was able to pick up lots for about 30 quid for a couple of logs.
Keep looking, keep networked and it'll flow in soon!