Bolivianos or USDollars?

just bring your bank card and some cash. your taxi driver can stop at an atm, and your hotel can hang onto your gear while you find an atm. ATMs give out local $$

bring small denomination US$ as you can spend these just about anywhere - as 1 says , then get BOLS out of the ATm or change at a bank or casa de cambia the next day. Some hotels will change money for you also so you should have no problems. A taxi from the airport should cost about 60 bols or 7.5us$ to the center

There is an exchange booth on the right hand side just outside the customs hall, but it may not be open a 8 p.m. on a Saturday evening. You could just walk through to the departures hall (the La Paz airport buiilding is quite small) where you will be able to draw Bolivianos from one of two or three ATMs. This might be better than asking your taxi to stop at an ATM on the street. But the exchange rate at an ATM is not as good as at a "casa de cambio" (currently us$1:Bs.7.82, and still going down).
But as tunabagel says, small denomination US$ bills will usually be accepted.

One other idea -- where's your (U.S.) departure city... Miami? Dallas? NYC? You might be able to search out one of those Thomas Cook exchange booths (assuming they have BOLs on hand and you don't mind getting broomsticked with the exchange rate).

the "casa de cambio" (or really... woman sitting in a little wooden box) will be open when you arrive. The exchange rate there was 7.84 Bs = $1 a week ago when my parents arrive on the 8pm flight from Miami.

In La Paz, there are Casas de Cambio on Camacho.. open normal business hours during the week, and until 12:30pm on saturdays. And if you speak any spanish, the security guards will probably try to flirt with you.

I switched my checking account to HSBC, which will give you whatever currency you want before you leave and their fee is extremely small. Very convenient. It's just then deducted from your checking balance. It takes only a day or two. (I'm going to La Paz 7/27/07 - hope I have fun).