Ta for the note and continued thoughts along your quest - it will be interesting to read how things go for you.
Bordeaux is perhaps the easiest of wine regions to get hold of aged examples - with it being both a recognised stayer in the cellar, plus that it's been the favourite for investors for years. The downside of this, is sometimes the prices outstrip what I'd consider the value of the wine - investors want their return!
I'm lucky in knowing someone who trades (B2B) in wine, which is a help. However there should be plenty of wines come up at auction (better perhaps to seek out a local or provincial auction, than the high profile big bucks ones). Pricing is easy via wine-searcher and plenty of info available in print or on the web around how older vintages are doing (Broadbent is a great read for such stuff, but there are loads of other worthwhile critical opinions). Some retail shops will have a decent selection, but it's a matter of finding which ones. Expect to pay ~ 30-40% more than auctions normally (though with auction prices as they have been here recently, the gap is a lot closer, but clearly that can't last).
Finally, there's a fair chance you know folk who have got plenty of mature claret in their cellars. Whether it's offering a few low key wine trades, or just persuading them to bring some along to an informal tasting, that might be the best way of getting access to them.
Drink coffee, do stupid things faster