If one is in pursuit of restaurant meal toys, from McDonald's or any other fast food establishment, they can literally be found by the bucketful at Goodwills and Salvation Armys, or whatever your local brand of thrift store is. Often, you'll find toys still sealed in their plastic bags, and they all go for around 25 cents each. Ebay is also a good, cheap resource -- it's rare that anything produced after, say, '92 has much value. These companies produce millions upon millions of these toys for each promotion, and with everyone saving and collecting them, it's uncommon for any of them to be highly sought after (except for maybe the beanie baby insanity from awhile back).
However, if it's the actual meal you're after, the real solution is: Don't be embarassed. Who cares? The staff of a McDonald's is used to seeing odder things than an adult ordering a kid's meal for themself. A person working the register is probably on auto-pilot while taking your order, but actually daydreaming of what they're going to do after work. If you look embarassed or act oddly, you'll end up drawing unnecessary attention to yourself.