One year it's musical toothbrushes, another year it's clocks powered by potatoes. Each holiday season has its "hot items" that show up in many catalogues.

Here are some of this year's unusual but popular items (at least with the catalogue houses) along with one of the many places each can be found.

*Trivia Bits: Cheddar cheese crackers with trivia questions and answers inside. No preservatives. 56 crackers per tin. $5.98 (plus $2.25). The Lighter Side, 35075 Automation Drive, Mt. Clemens, Mich. 48043-4385.

*"I Don't Want to Grow Up": Some shirts say it slightly differently, but the message is the same. Cotton/poly T-shirt in adult sizes only, S-M-L-XL. $11.95 (plus $3.45). Giggletree, Winterbrook Way, Meredith, N.H. 03253.

*Presentational Bottles: Baby bottle of molded lead crystal and silverplate. Holds five ounces, $19.95 (plus $3.95). Potpourri, Dept. 129, 204 Worcester St., Wellesley, Mass. 02181.

*No Parking sign: 12-by-18-inch regulation size sign warns, "Don't Even THINK of Parking Here." Enamel on steel, $9.95 (plus $1). Lincoln House, 300 Greenbrier Rd., Summerville, W. Va. 26651.

*Bear feet: Fun fur slippers have printed pads on soles cloth claws. Children's sizes $14.95, men's and women's $19.95 (plus $2.95). Early Winters, 110 Prefontaine Pl. S., Seattle, Wash. 98104-2695.

*365 Cats calendar: There's scarcely a catalogue without it. Kitty picture, Kitty fact, etc., for each and every day of 1986. $8 (plus $2.75). Casual Living, Kent Road, P.O. Box 1078, New Milford, Conn. 06776.