Oh, Dianne, I hope you find them soon. But, you can still get your return done. Here are some suggestions (This is NOT tax advice as I'm not a tax expert--just some ideas that might help). If you have someone else do your taxes for you, they can probably help you out with more & better tips, too!
If you pay for things by check, debit or credit card, review your statements & try to remember what you bought where & recreate your expense record. This will be easier if you use a separate account for business expenses, of course.
If you don't have a separate account, it will be harder, but if you see you spent $100 in March at Best Buy, it will trigger your memory that that's your new hard drive, for instance. You might lose out on some smaller deductions this way if you typically pick up copy paper at Target along with your groceries, though.
You can also "look around"--what did you buy this year for your business? If you bought a new monitor, but don't remember what you paid, do some online price checking.
It does help to keep a monthly income/expense spreadsheet for many reasons, but this is one instance it could be a big help! If you have one, sweet. If not, think about starting one now for 2010.
Also, review your last year or last few years' tax returns -- if you've been running your business for awhile, your expense trends might be similar. For instance, if you have spent $250 for web hosting for two years in a row, it's a pretty good bet you probably spent about the same.
Some companies can also pull your information for you if you are a regular customer & they keep the records.
Also, if you have any bills where you get online invoices, you can usually still find those--from the company or even in your email archives. Ditto if you bought business stuff online. I buy a lot of business books from Amazon and I can access my orders from them easy-peasy.
And, your calendar can help for some expenses--networking events, for instance--I write these in my calendar so if I needed to determine how much I spent on networking, I could get really close that way. I also record my mileage in there, so that's where I get my auto expense deduction (if you use the standard mileage deduction, you don't need receipts.)
These things can help you get your taxes done if you can't find the receipts soon, AND, you can always file an amended return when you find them! (Well, you have a certain time limit that you can find on the IRS website, but it's pretty long, I think.)
And, last but not least, if you get busy doing these things, odds are you'll remember where the receipts are when you stop thinking about it! LOL--that's how it goes for me anyway!
Much luck on finding them ASAP!